33    452 


H 


, 
NHB 


COMMERCIAL  DYNAMO 
DESIGN 


ORIGINAL   PAPERS 


ON 


COMMERCIAL  DYNAMO 
DESIGN 


BY 

WILLIAM  L.  WATERS,  M.E.,  E.E. 

Engineer  in   Charge  of  High  Speed  Machinery,  Westinghouse  Electric 
Manufacturing  Co. ;  formerly  Chief  Engineer,   National  Elec- 
tric Co. ,  and  Consulting  Engineer,  Westinghouse  Air 
Brake  and   Canadian    Westinghouse  Cos. 


FIRST  EDITION 


NEW  YORK 

JOHN  WILEY  &  SONS 

LONDON:  CHAPMAN  &  HALL,  LIMITED 

1911 


K^' 


COPYRIGHT  1911 

BY 
WILLIAM  L.  WATERS 


"//  is  not  knowledge,  but  ignorance,  that  begets 
confidence."  CHARLES  DARWIN 

' '  /  hold  every  man  a  debtor  to  his  profession  ;  from 
the  which,  as  men  of  course  do  seek  to  receive  countenance 
and  profit,  so  ought  they  of  duty  to  endeavor  themselves 
by  way  of  amends  to  be  a  help  and  ornament  thereunto.  " 

FRANCIS  BACON 


343G52 


INTRODUCTION 


Commercial  Engineering. — The  nineteenth  century  was  nota- 
ble for  the  achievements  of  the  engineer,  and  there  is  little  doubt 
that  the  men  responsible  for  this  pioneer  work  were  engineers  in 
the  broadest  sense  of  the  word.  They  were  engineers  in  their 
ability  to  manipulate  the  forces  and  materials  of  nature ;  but  they 
were  also  far-sighted  and  level-headed  men  of  affairs  in  their  ap- 
preciation of  the  economic  value  of  their  work,  and  in  the  man- 
agement of  their  projects.  These  men,  by  their  genius  and 
enterprise,  and  by  their  hard  common  sense  and  solid  achieve- 
ment, forced  the  world  to  recognize  the  engineer  as  destined 
to  play  a  leading  part  in  the  future.  Public  attention  became 
focussed  on  the  engineer,  and  numbers  of  men  were  attracted 
to  the  profession,  cither  through  natural  inclination  or  through 
hope  of  profit.  But  as  was  to  be  expected,  a  large  percentage 
of  these  men  were  imperfectly  educated,  and  possessed  only  a 
specialized  talent  in  certain  directions,  without  any  of  the  broad- 
minded  comprehensive  spirit  of  the  pioneers.  This  change  soon 
reacted  on  public  opinion,  and  the  engineer  lost  his  high  stand- 
ing; so  that  the  public  and  the  world  of  commerce  came  to  re- 
gard him  as  a  specialized  "crank,"  a  species  of  high-grade  artisan, 
who  though  useful  enough  in  his  place,  was  devoid  of  any  ability 
to  take  a  sane  and  comprehensive  view  of  a  situation,  or  to  man- 
age an  undertaking  in  which  he  might  be  interested.  The  com- 
mercial world,  though  distrusting  the  ability  of  an  engineer  to 
manage  his  enterprises,  came  to  realize  more  and  more  their 

iii 


iv  INTRODUCTION 

economic  value  and  financial  possibilities.  The  result  of  this  was, 
that  the  enterprises  which  the  engineering  world  proposed, 
gradually  came  to  be  exploited  by  business  men,  who  had  to 
a  greater  degree  the  confidence  of  the  financial  powers.  These 
men,  who  attempted  to  manage  such  undertakings,  were  usually 
altogether  ignorant  of  the  work  they  took  up,  and  it  is  not  sur- 
prising that  this  development  produced  indifferent  results.  En- 
gineers, being  effectively  cut  off  from  the  commercial  side  of  their 
work,  became  narrower;  while  the  increasing  specialization  left 
the  commercial  men  in  still  greater  ignorance  of  the  products  they 
were  handling,  and  their  management  became  more  and  more 
wasteful.  The  inefficiency  of  this  arrangement  has  gradually  be- 
come evident  to  all,  and  it  is  now  generally  recognized  that  the 
executive  and  commercial  head  of  any  large  engineering  enter- 
prise must  posess  some  engineering  knowledge.  It  is  also  realized 
that,  given  an  opportunity  for  a  broader  education,  an  engineer 
can  be  readily  trained  to  become  an  efficient  and  high-grade  com- 
mercial executive,  while  it  is  almost  impossible  to  instil  an  en- 
gineering knowledge  into  one  whose  training  has  been  restricted 
to  the  commercial  world.  As  a  result  of  this,  engineers  are  being 
given  opportunities  to  obtain  a  broader  commercial  education, 
and  it  is  recognized  in  high  financial  quarters  that  in  the  future 
they  will  have  to  depend  on  the  engineering  profession  for  high- 
grade  operating  men  in  all  engineering  enterprises.  Already  we 
find  engineers  at  the  head  of  a  number  of  the  large  concerns,  and 
it  is  probable  that  the  time  is  not  far  distant  when  it  will  be 
the  exception  to  find  the  operation  of  an  engineering  under- 
taking in  the  hands  of  any  other  than  an  engineer. 

Mr.  John  I.  Beggs. — It  was  my  good  fortune  to  work  under 
Mr.  John  I.  Beggs,  one  of  the  first  and  one  of  the  most  prominent 
of  the  commercial  engineering  executives  in  this  country,  at  a 
time  when  I  was  so  interested  in  purely  engineering  problems 
that  I  was  in  danger  of  drifting  away  from  broader  interests. 
For  twenty-five  years  in  active  touch  with  the  electrical  engineer- 
ing industry,  as  a  manufacturer,  or  as  an  organizer  and  operator 
of  public  service  corporations,  obtaining  his  engineering  and 


INTRODUCTION  v 

commercial  knowledge  through  hard  personal  experience,  Mr. 
Beggs  was  pre-eminently  fitted  to  guide  and  encourage  the 
youthful  engineer.  Continually  improving  and  rendering  com- 
mercially practicable  much  of  the  apparatus  used  in  connection 
with  electric  lighting  or  railway  interests,  and  doing  this  with 
the  purpose  of  obtaining  results,  rather  than  of  being  accorded 
credit  or  public  recognition,  he  was  one  of  the  first  to  realize 
that  the  engineer  was  destined  to  become  the  dominant 
factor  in  large  engineering  corporations.  .Giving  his  young 
engineers  a  free  hand  in  all  branches  of  commercial  and 
engineering  work,  only  guiding  and  checking  them  where  neces- 
sary, he  developed  organizations  and  men  in  a  way,  which,  while 
it  made  the  men  his  grateful  and  enthusiastic  admirers,  estab- 
lished his  reputation  as  one  of  the  foremost  organizers  in  the 
engineering  industry,  and  left  his  mark  on  the  methods  and 
apparatus  of  to-day.  In  inscribing  these  few  papers  to  Mr. 
John  I.  Beggs,  I  am  only  taking  an  opportunity  of  express- 
ing my  indebtedness,  and  my  appreciation  of  the  education  it 
was  to  be  in  touch  with  him.  I  have  entitled  the  papers 
"Commercial  "  Design,  because  engineering  questions  of  purely 
academic  interest  are  made  subservient  to  those  broader 
issues,  where  design  and  operation  must  be  considered  in  their 
relation  to  the  commercial  success  and  future  of  the  undertak- 
ing. It  is  in  this  feature  of  my  work  that  I  have  to  ac- 
knowledge the  influence  of  Mr.  Beggs,  and  to  state  that  I  have 
in  a  great  measure  to  thank  him  for  any  small  success  I  may 
have  had. 

History  of  Papers. — These  papers,  which  have  been  written 
at  different  periods  during  the  past  six  years,  are  the  result 
of  fifteen  years'  experience  as  a  designing  and  manufacturing 
engineer  in  some  of  the  most  important  electrical  manufacturing 
concerns  in  Europe  and  America.  They  cover  various  subjects 
relating  to  the  design  of  electrical  power  machinery,  in  which  I 
happened  to  have  been  interested  at  different  times,  and  they 
are  reproduced  here  in  the  hope  that  they  may  be  more  accessible, 
to  any  whom  they  may  interest,  than  they  would  be  in  the  pro- 


vi  INTRODUCTION 

ceedings  of  the  various  institutions.  The  student  will  find  there 
is  no  lack  of  treatises  on  dynamo  design  by  capable  men,  and 
this  book  makes  no  pretense  to  be  a  comprehensive  work.  It  is 
merely  offered  as  a  supplement  to  such  treatises,  as  a  series  of 
articles,  which  written  by  one  who  is  actively  engaged  in  prac- 
tical manufacturing,  may  cover  a  few  points  of  special  interest 
to  the  student  or  designing  engineer,  that  could  not  be  satis- 
factorily covered  in  a  more  general  work. 

December,  1910. 


CONTENTS 

PAGE 

INTRODUCTION         ....                 iii 

COMMERCIAL  ALTERNATOR  DESIGN 1 

DOUBLE  CURRENT  GENERATORS  IN  THEIR  RELATION  TO  DOUBLE  CURRENT 

SUPPLY 23 

PREDETERMINATION  OF  SPARKING  IN  DIRECT  CURRENT  MACHINES        .  29 
ROTARY    CONVERTERS  AND    MOTOR-GENERATORS           .        .        .        .45 

SHUNT  AND  COMPOUND  ROTARY  CONVERTERS  FOR  RAILWAY  WORK        .  69 

THE  NON-SYNCHRONOUS  OR  INDUCTION  GENERATOR  IN  POWER  STATION 

WORK 75 

MODERN  DEVELOPMENTS  IN  SINGLE  PHASE  GENERATORS       .        .        .  103 

PERFORMANCE  SPECIFICATIONS  AND  RATINGS 113 

INPUT-OUTPUT  EFFICIENCY  TESTS 119 

DIRECT  CURRENT  TURBO  GENERATORS  123 


[Presented  before  the  American  Institute  of 
Electrical  Engineers,  June  29,  1903.] 


COMMERCIAL  ALTERNATOR  DESIGN 


THE  design  of  alternators  has  been  treated  times  without 
number,  but  usually  the  commercial  element  in  the  design,  i.e., 
the  relation  of  factory  cost  to  selling  price,  has  been  ignored. 
An  engineer  has  been  defined  as  a  man  "who  can  do  for 
one  dollar  what  any  fool  can  do  for  two,"  and  as  this  definition 
applies  in  connection  with  machine  design,  the  man  who  can 
design  the  cheaper  machine  to  satisfy  a  given  specification  is 
the  better  designing  engineer. 

Speaking  generally,  there  is  no  type  of  alternator  that  will 
compare  with  the  internal  revolving  field  construction,  in  which 
each  pole  carries  a  separate  field  coil  of  edge-on  copper  strap. 
The  revolving  armature  is  cheaper  for  high  frequencies  and  low 
voltages,  the  inductor  type  is  good  for  small  60  cycle  high- 
speed machines,  while  the  disc  alternator  with  no  iron  in  the 
armature  is  an  excellent  machine  for  high  frequencies.  But 
these,  though  sufficiently  satisfactory  in  their  own  limited  field, 
do  not  compare  with  the  revolving  field  type  for  general  work. 

Fundamental  Types  of  Construction. — The  revolving  field 
alternator  took  its  present  form  about  1892  when  Mr.  C.  E.  L. 
Brown  designed  some  alternators  which  were  practically  modern 
machines;  while  in  1893  Mr.  S.  Z.  de  Ferranti*  installed  some 

*  I  understand  from  Mr.  C.  E.  L.  Brown  that  the  Brown  Boveri  Maschin- 
enfabrik  deserve  to  some  extent  the  credit  for  the  design  of  the  Portsmouth 
alternators,  though  Mr.  Ferranti  was  the  first  to  use  edge-on  copper  strap  for 
field  magnets. 

1  1 


COMMERCIAL  ALTERNATOR  DESIGN 


FIG.  1. 


COMMERCIAL  ALTERNATOR  DESIGN 


FIG.  2. 


4  COMMERCIAL  ALTERNATOR   DESIGN 

210  K.W.  alternators  at  Portsmouth,  England,  which  were  of 
similar  design  to  those  of  Mr.  Brown.  Before  that  date  alter- 
nators of  this  type  were  of  a  clumsy  amateur  design,  and  their 
performance  wras,  generally  speaking,  very  poor.  Strangely 
enough,  these  two  engineers,  after  bringing  out  a  high  grade 
design,  apparently  abandoned  further  development,  and  their 
machines  to-day  are  almost  identical  with  their  machines  of  ten 
years  ago. 

It  has  taken  the  different  manufacturing  concerns  a  long 
time  to  recognize  the  superiority  of  the  Brown  and  Ferranti 
type  for  standard  work,  and  it  is  practically  only  during  the 
last  three  or  four  years  that  this  type  has  been  generally  adopted. 
The  result  is  that,  except  for  a  few  minor  details,  the  construc- 
tion of  these  alternators  has  been  improved  very  little  since 
they  were  first  introduced;  while  the  excellence,  from  a  commer- 
cial point  of  view,  of  the  electrical  design  of  Mr.  Brown's  early 
machines  seems  hardly  recognized  even  yet  by  some  engineers; 
and  we  have  alternators  on  the  market  to-day  which  are  for  a 
given  performance  decidedly  more  expensive  than  those  which 
he  designed  ten  years  ago. 

The  armature  frame  in  the  Brown  type  of  machine  was 
only  a  skeleton  cast  iron  frame  for  clamping  the  laminations  to- 
gether, and  was  provided  with  large  ventilating  holes;  while  the 
ends  of  the  armature  coils  stood  out  from  the  laminations,  quite 
free  and  exposed  to  the  full  windage  of  the  magnet- wheel.  The 
numerous  holes  gave  excellent  cooling  effect,  but  they  reduced 
the  strength  and  stiffness  of  the  frame,  so  that  the  armature 
had  to  be  stiffened  by  a  series  of  tie-rods  or  struts.  This  con- 
struction, which  saves  material  at  the  expense  of  labor,  has 
become  standard  with  German  and  Swiss  firms,  though  on 
account  of  its  unsightly  appearance  it  has  never  found  favor 
in  this  country.  This  type  of  alternator,  shown  in  Fig.  1,  has 
retained  practically  its  original  form  up  to  the  present  time  and 
developments  that  have  taken  place  have  been  mostly  in  the 
Ferranti  type. 

American  and  English  engineers  have  followed  the  Ferranti 


COMMERCIAL  ALTERNATOR   DESIGN 


FIG.  3. 


6  COMMERCIAL  ALTERNATOR   DESIGN 

type  shown  in  Fig.  2,  and  made  the  armature  frame  stiff  enough 
to  stand  without  bracing.  The  trouble  with  this  construction 
was  originally  the  poor  ventilation  of  the  armature.  Ventilating 
spaces  were  either  not  used,  or  if  they  were,  there  was  no  proper 
circulation  of  air  through  them.  The  end  connections  on  the 
armature  winding  and  the  ends  of  the  coils  were  packed  tight 
together,  or  were  closed  in  by  cover-plates  permitting  no  ventila- 
tion. Thus,  the  armature  winding  was  usually  the  hottest 
part  of  the  machine;  so  that  even  allowing  temperature  rises  of 
45°  C.,  these  machines  could  not  be  rated  as  they  should,  solely 
on  account  of  the  poor  ventilation.  When  American  and 
English  engineers  took  up  the  revolving  field  type  of  alternator, 
the  badly  ventilated  Ferranti  type  was  adopted,  and  the  great 
importance  of  ventilation  was  not  recognized,  so  that  the  de- 
velopment of  alternator  design  in  America  and  England  has  been 
comparatively  slow. 

The  improvement  in  ventilation  which  has  recently  taken 
place  in  this  type  of  alternator  is  really  the  greatest  forward  step 
that  has  been  made,  and  it  has  given  the  designer  immense  help 
in  increasing  the  output  of  machines. 

Fig.  3  shows  an  old,  badly  ventilated  armature,  while  Figs.  4 
and  8  show  a  more  up-to-date  well  ventilated  machine.  In  Figs. 
4  and  8  it  will  be  seen  that  where  the  ends  of  the  armature  coils 
cross  one  another  they  are  separated  by  an  air  space,  and  that 
the  end  covers  are  provided  with  ventilating  holes,  to  allow  a 
circulation  of  air  around  the  coils;  thus  the  armature  winding, 
instead  of  being  the  hottest  part  of  the  machine,  becomes  the 
coolest.  The  armature  core  is  well  provided  with  vent  spaces 
both  at  the  centre  and  at  the  ends,  and  the  air  passing  through 
these  vents  is  free  to  escape  at  the  back  of  the  core.  This  type 
of  armature  coil  has  the  additional  advantages  that  if  lightning 
gets  into  the  machine  or  a  coil  is  burnt  out,  the  damage  is  con- 
fined to  one  coil,  so  that  we  do  not  have  half  a  dozen  burnt  out 
as  usually  happens.  Also,  all  the  coils  on  the  armature  are  alike 
and  made  on  the  same  former,  so  that  the  question  of  spare 
coils  is  simplified.  The  difference  in  the  cooling  effect  between 


COMMERCIAL  ALTERNATOR  DESIGN 


FIG.  4. 


8  COMMERCIAL  ALTERNATOR   DESIGN 

these  different  designs  may  not  seem  to  be  much  on  paper,  but 
it  results  in  the  difference  between  a  temperature  rise  of  45° 
and  one  of  25°,  on  actual  test.  It  means  that  we  need  only 
take  into  consideration  efficiency  and  regulation  in  designing  a 
machine,  knowing  well  that  if  these  are  satisfactory  we  can 
guarantee  a  temperature  rise  of  25°,  even  on  low-speed  machines. 
When  designing  any  machine  we  have  the  choice  of  taking  a 
large  diameter  and  making  the  machine  short,  or  of  taking  a 


FIG.  5. 


small  diameter  and  making  the  machine  long.  The  difference  in 
the  cooling  effect  between  these  two  is  obvious  from  Figs.  5  and  6. 
In  Fig.  5  the  machine  is  small  in  diameter  and  long,  the  poles  are 
close  together  and  the  winding  crowded,  while  all  the  heat  from 
the  field-coils  has  to  be  dissipated  from  the  small  exposed  surface 
at  the  ends  of  the  coils.  In  Fig.  6  the  alternator  is  large  in 
diameter  and  short,  and  practically  the  whole  surface  of  the  field- 
coil  is  available  for  cooling.  In  addition,  the  field  coils  being 


COMMERCIAL  ALTERNATOR   DESIGN  9 

separated  more  from  one  another  and  the  peripheral  speed 
higher,  the  cooling  effect  on  the  armature  is  much  greater.  The 
machine  in  Fig.  6,  being  built  on  a  large  diameter,  will  require 
heavier  castings  and  present  greater  difficulties  in  handling,  but 
the  fact  that  the  designer  is  not  restricted  by  the  temperature 
rise,  gives  him  so  much  more  latitude,  that  he  will  easily  offset 
this  slight  extra  expense  by  a  cheaper  design  generally,  and  will 
in  addition  have  a  much  cooler  machine.  The  difference  in 


FIG.  6. 


cooling  effect  between  a  construction  with  armature  and  mag- 
nets as  shown  in  Figs.  3  and  5  and  one  with  armature  and 
magnets  as  shown  in  Figs.  4  and  6  is  so  obvious,  that  it  is  quite 
surprising  to  find  the  poorly  ventilated  type  still  on  the  market. 
The  only  inference  to  be  drawn  is  that  the  firms  building  them 
have  not  given  the  subject  due  thought. 

Specification.— In  the  electrical  part  of  the  design,  the  first 
thing  to  be  decided  is  the  specification  to  which  the  machine 


10  COMMERCIAL  ALTERNATOR  DESIGN 

is  to  be  built.  The  firm  with  which  I  am  connected  has  adopted 
as  standard: 

A  temperature  rise  of  35°  C.  on  a  continuous  full  load  run; 

A  temperature  rise  of  50°  C.  on  50  per  cent  over-load  for  two 
hours; 

A  regulation  of  5  to  7  per  cent  on  non-inductive  load,  accord- 
ing to  the  size  of  the  machine; 

And  gives  a  guarantee  that  all  machines  will  without  damage 
give  continuously  25  per  cent  current  over-load  at  zero  power 
factor. 

Detail  Design. — Given  the  specifications,  we  have  next  to 
decide  what  diameter  we  shall  make  the  machine  What 
magnetic  densities  to  take  in  the  iron?  What  current  density  in 
the  conductors?  What  percentage  of  the  pole  pitch  shall  the 
pole  face  be?  What  air  gap?  Of  course,  these  questions  can 
only  be  answered  off-hand  as  the  results  of  experience.  But 
generally  speaking  we  can,  after  a  few  trials,  decide  on  the  best 
design.  We  have  only  to  consider  the  efficiency,  the  regulation, 
and  the  cost;  as  with  a  good  design  the  temperature  need  not  be 
considered. 

The  efficiency  of  a  machine  within  ordinary  limits  practically 
depends  on  the  magnetic  densities  in  the  iron  and  the  current 
densities  in  the  copper.  The  higher  the  densities,  the  cheaper  and 
the  less  efficient  the  machine.  The  copper  loss  in  the  armature  is 
usually  between  1  and  2  per  cent.  Apart  from  the  efficiency  this 
is  decided  by  the  regulation,  because  if  we  allow  only  5  to  7  per 
cent  voltage  drop  on  P  F  =  1,  we  cannot  well  have  more  than 
2  per  cent  of  this  as  C  R  drop.  This  means  that  in  low-speed 
machines  with  a  large  number  of  poles,  the  current  density  in  the 
armature  is  very  low,  while  in  high-speed  machines  with  few 
poles  the  current  density  can  be  much  higher.  In  practice  it 
varies  from  1200  to  3000  amperes  per  square  inch.  The  iron 
densities  do  not  vary  much  in  standard  machines,  as  the  most 
economical  densities  are  very  fairly  constant  and  independent  of 
the  speed;  and  any  attempt  to  obtain  higher  efficiencies  by  de- 
creased iron  densities  will  increase  the  cost  rapidly. 


COMMERCIAL   ALTERNATOR  DESIGN  11 

The  best  ratio  of  pole  face  to  pole  pitch  is  largely  a  matter  of 
opinion.  If  it  is  large,  say  70  per  cent,  then  the  E.M.F.  coeffi- 
cient (the  Kapp  coefficient)  is  reduced,  and  the  total  flux  of  the 
machine  increased  and  hence  the  magnets  made  heavier.  On 
the  other  hand,  with  a  wide  pole  face  we  have  more  teeth  to 
carry  the  flux,  and  for  a  given  tooth-density  the  machine  is 
shorter.  But  the  armature  core-plates  are  correspondingly 
deeper,  so  that  the  only  saving  is  a  slight  decrease  in  the  length 
of  mean  turn  of  the  armature  winding.  The  larger  the  per- 
centage of  pole  face  to  pole  pitch  the  greater  the  magnetic  leak- 
age, and  to  a  certain  extent  the  less  the  synchronizing  power  of 
the  alternator.  So  that  there  is  little  to  be  gained  by  much 
variation  of  this  ratio,  and  it  seems  advantageous  to  keep  it 
!ow,  say  between  60  and  65  per  cent. 

The  air  gap  is  decided  by  the  regulation  of  the  machine.  An 
immense  amount  has  been  written  on  various  theoretical  methods 
of  calculating  the  regulation  of  alternators,  but  broadly  speaking 
the  regulation  depends  on  the  ratio  of  the  ampere-turns  on  the 
armature  to  the  ampere-turns  for  the  air-gap.  In  an  alternator 
the  armature  conductors  are  cut  by  magnetic  lines  due  to  the 
armature  current,  i.e.,  the  armature  self-induction  flux;  and  by 
magnetic  lines  due  to  the  magnet  current.  But  the  self-induction 
of  the  armature  varies  with  the  magnitude  of  the  current  in 
the  armature  and  magnets,  and  with  their  relative  position; 
while  the  useful  flux  due  to  the  magnets  varies  with  the  current 
in  the  armature  and  field,  on  account  of  the  varying  perme- 
ability of  the  iron  and  the  magnetic  leakage.  So  it  is  obvious 
the  conditions  to  be  taken  into  account  are  so  complicated  that 
it  becomes  quite  impossible  to  treat  them  theoretically,  without 
making  so  many  assumptions  that  the  results,  even  when 
obtained,  cannot  be  directly  applied.  What  a  designer  has 
to  do  is  to  work  theoretically  through  a  few  simple  special  cases 
himself  and  the  results  will  give  him  an  idea  on  what  lines  to 
work.  Then  by  means  of  experimenting  on  a  number  of  ma- 
chines he  can  develop  an  empirical  method  for  calculating  the 
regulation.  Afterwards  as  he  gets  more  and  more  experience 


12  COMMERCIAL  ALTERNATOR   DESIGN 

with  alternators,  he  introduces  further  refinements,  and  taking 
the  regulation  curves  obtained  by  empirical  methods  he  corrects 
them  a  little  by  eye. 

Speaking  generally  from  a  designer's  point  of  view,  an  alter- 
nator should  be  calculated  for  a  certain  regulation  on  a  low 
powder-factor  load,  say  P  F  =  0.  For,  if  the  machine  is  satis- 
factory on  low  power-factors,  it  will  be  satisfactory  for  non- 
inductive  loads,  while  the  converse  is  not  true.  Other  things 
being  equal,  the  larger  the  air-gap  the  better  the  regulation 
on  the  low  power-factors.  But  the  leakage  coefficient  of  the 
machine  is  an  important  factor,  and  this  increases  with  the  air- 
gap.  This  coefficient  is,  of  course,  taken  into  account  in  drawing 
the  no  load  saturation  curve;  but  we  have  to  remember  that  on 
full  load  of  low  power-factor  the  leakage  coefficient  is  much 
increased  (the  leakage  is  often  doubled),  on  account  of  the 
additional  ampere-turns  required  on  the  magnets  to  overcome 
the  demagnetizing  ampere  turns  on  the  armature.  So  that  if 
the  leakage  coefficient  is  already  high,  and  if  the  density  in  the 
magnet  iron  is  also  high,  we  run  a  considerable  risk  of  saturating 
the  magnet  circuit  so  that  we  cannot  obtain  the  rated  voltage  on 
loads  of  a  low  power-factor.  It  was  this  trouble  that  caused 
inductor  machines  to  become  obsolete  for  low  power-factor  loads, 
.as  they  are  particularly  sensitive  to  leakage  and  are  always 
worked  at  high  densities.  Speaking  generally,  if  the  no  load 
leakage  coefficient  of  a  machine  is  higher  than  1.25,  and  if  the 
density  in  the  magnets  is  greater  than  100,000  lines  per  sq.  in., 
the  designer  has  to  be  very  careful  or  he  will  be  in  difficulties. 

The  regulation  on  non-inductive  loads  is  not  affected  by  the 
length  of  the  air-gap  to  the  same  extent  as  the  regulation  on  low 
power-factors.  So  machines  which  are  only  intended  for  lighting 
or  rotary  converter  work  usually  can  be  economically  designed 
with  a  smaller  air-gap  and  higher  densities  than  machines  for 
motor  work. 

In  Europe  practically  every  alternator  sold  has  to  operate 
motors,  so  that  the  regulation  either  for  P  F  =  0.8  or  for  P  F  = 
0  has  to  be  guaranteed.  In  America,  on  account  of  the 


COMMERCIAL  ALTERNATOR   DESIGN  i:J 

patent  situation,  induction  motors  are  used  only  to  a  limited 
extent,  and  as  a  result  of  this  it  has  become  standard  practice 
to  sell  machines  on  a  regulation  guarantee  for  non-inductive, 
rather  than  for  inductive  loads.  This  is  very  unsatisfactory. 
Almost  every  load  that  an  alternator  has  to  carry  is  to  a  certain 
extent  inductive,  e.g.,  arc  lamps,  transformers  on  light  loads, 
rectifiers,  induction  motors,  and  synchronous  motors,  unless  the 
excitation  is  carefully  adjusted.  And  as  the  regulation  of  an 
alternator  on  P  F  =  0.9")  is  usually  about  twice  as  bad  as  on 
P  F  --=  1,  it  is  obvious  that  a  more  satisfactory  guarantee  would 
be  to  give  regulation  on  inductive  loads.  Practically  the  only 
exception  to  this  is  the  case  of  an  alternator  for  use  exclusively 
for  running  rotary  converters.  And  even  with  a  compound- 
wound  rotary  converter  and  an  inductive  line,  the  power-factor 
is  usually  low  and  the  current  lagging  for  light  loads;  while  if 
the  rotaries  have  to  be  started  from  the  alternating  current  side, 
a  generator  with  poor  regulation  on  low  power-factors  is  very 
noticeable  and  may  give  trouble. 

It  is  extremely  difficult  to  measure  the  regulation  for  P  F  --=  1 
on  any  machine  with  good  regulation,  while  on  a  large  machine 
it  is  practically  impossible.  The  result  can  only  be  figured  as 
the  difference  between  two  large  quantities,  and  there  are  so 
many  disturbing  features  that  the  result  when  obtained  is  not 
very  accurate.  On  the  other  hand,  it  is  quite  easy  to  measure 
the  regulation  on  a  very  low  power-factor  by  loading  on  to  a 
second  machine  running  as  a  synchronous  motor,  the  first  one 
operating  as  a  generator,  and  varying  the  excitation  of  the 
motor  and  generator  till  full  load  current  is  flowing  at  full-load 
voltage.  The  power-factor  in  such  a  test  will  be  very  low  and 
can,  with  sufficient  accuracy,  be  taken  zero. 

Alternators  can  be  designed  so  as  to  satisfy  a  close  regulation 
specification  for  non-inductive  loads  and  yet  be  almost  worthless 
for  carrying  loads  of  low  power-factor.  So  that  as  such  machines 
can  be  made  cheaper  than  if  they  were  required  to  give  a 
reasonable  regulation  on  inductive  loads,  there  is  a  temptation 
for  manufacturers  to  take  advantage  of  the  fact  that  the  regula- 


14  COMMERCIAL   ALTERNATOR    DESIGN 

tion  is  only  guaranteed  on  P  F  =  1,  to  install  one  of  these 
cheaper  machines.  It  is  probably  this  fact  which  is  responsible 
for  the  number  of  alternators  having  poor  regulation  on  low 
power-factors,  that  have  been  installed  in  this  country.  It 
would  certainly  be  an  advantage  from  the  customer's  point  of 
view,  and  probably  in  the  end  from  that  of  the  manufacturer 
also,  if  the  regulation  were  guaranteed  for  a  load  of  low 
power-factor.  This  would  make  it  necessary  from  the  com- 
mercial standpoint  to  alter  somewhat  the  lines  on  which  modern 
alternators  are  designed,  but  the  cost  of  the  machines  would 
not  necessarily  be  much  increased.  A  modern  alternator  gives, 
say,  7  per  cent  regulation  on  P  F  =  1,  and  22  per  cent  on  P 
F  =  0.8.  When  operating  with  a  normal  power-factor  of 
about  0.85,  and  a  regulation  of  about  17  per  cent,  it  does  not 
help  the  station  engineer  to  know  that  if  he  had  a  non-induct- 
ive load  he  would  have  good  regulation.  Such  a  machine  could 
be  re-designed  on  somewhat  different  lines,  so  as  to  have  6  per 
cent  regulation  on  P  F  =  1,  and  12  per  cent  on  P  F  --=  0.8, 
and  about  one  per  cent  lower  efficiency  without  increasing  the 
cost  appreciably.  Such  a  machine  would  be  much  more  satis- 
factory for  general  work  and  could  probably  be  sold  for  consid- 
erably more  than  the  machine  designed  only  for  work  on 
non-inductive  loads. 

Other  things  being  equal,  the  regulation  of  an  alternator  is 
better  the  more  saturated  the  magnet  circuit,  and  this  applies  to 
low  power-factor  loads  as  well  as  to  high.  It  can  be  considered 
simply  as  an  experimental  fact,  or  the  explanation  can  be 
accepted  that  the  voltage  drop  in  an  alternator  is  partly  due  to 
the  reaction  of  the  armature  ampere  turns,  and  that  the  effect 
of  a  definite  percentage  change  in  the  ampere-turns  is  less  when 
the  magnets  are  saturated  than  when  they  are  not.  Obviously 
the  part  of  the  magnetic  circuit  to  saturate  is  the  magnet  core, 
as  the  less  its  cross  section,  the  less  its  perimeter  and  the  less  the 
weight  of  the  magnet  copper.  In  the  type  of  magnet  shown  in 
Fig.  7,  it  is  impossible  to  saturate  the  pole  core,  and  the  large 
amount  of  iron  and  copper  necessary  always  makes  this  design 


COMMERCIAL   ALTERNATOR   DESIGN  15 

needlessly  extravagant.  It,  however,  possesses  the  advantage 
that  the  voltage  can  be  raised  25  or  30  per  cent,  if  desired,  to 
compensate  for  an  extraordinary  line  drop;  though  usually  it 
is  sufficient  if  an  alternator  is  capable  of  having  its  voltage 
increased  15  per  cent  when  carrying  full  load. 

When  designing  an  alternator  for  a  given  output  we  can  adopt 
either  a  strong  armature  and  a  weak  field,  or  a  weak  armature 
and  a  strong  field;  and  generally  speaking,  the  stronger  the 
armature,  the  cheaper  the  machine  but  the  worse  the  regulation. 
So  to  design  cheap  machines  with  good  regulation,  it  is  necessary 
to  take  advantage  of  everything  that  will  better  our  regulation, 
i.e.,  we  must  work  with  a  long  air  gap  and  we  must  saturate  our 
poles.  But  a  long  air  gap  results  in  large  leakage,  and  as  I 
pointed  out  before,  a  machine  with  large  leakage  and  saturated 
poles  is  the  most  difficult  machine  to  design.  To  make  a 
uniform  success  of  such  machines,  the  designer  must  have  had 
considerable  experience  with  the  type  of  alternator  in  question, 
and  must  be  a  very  careful  worker.  In  fact,  when  I  first  began 
designing  alternators  I  was  told  to  put  plenty  of  iron  and  plenty 
of  copper  into  the  magnets,  and  that  if  I  did  this  I  would  be  safe. 
I  think  for  a  beginner  the  advice  is  good  and  that  he  could  not 
do  better  than  commence  with  a  conservative  and  simple  design 
like  that  shown  in  Fig.  7.  But  for  a  designer  who  has  had 
considerable  experience,  it  is  well  to  figure  more  closely,  as  there 
is  quite  20  per  cent  in  the  cost  to  be  saved  by  so  doing. 

Suppose  we  have  decided  to  adopt  a  large  air-gap  and  yet 
wish  to  keep  down  the  leakage.  There  are  several  things  which 
will  help  us  in  this,  but  making  the  pole  pitch  large  and  decreas- 
ing the  length  of  the  magnet  pole  are  the  most  important. 
Adopting  a  large  pole  pitch  results  in  making  the  machine 
larger  in  diameter,  and  shorter.  Beyond  certain  limits  this  in- 
creases the  cost,  and  it  is  a  question  to  be  decided  by  the 
designer  as  to  when  the  advantages  obtained  from  the  larger 
pole  pitch  are  offset  by  the  increased  diameter  and  weight  of  the 
castings.  Decreasing  the  length  of  the  magnet  pole  core  reduces 
the  leakage.  Decreasing  this  length  also  results  in  decreases 


16 


COMMERCIAL   ALTERNATOR   DESIGN 


the  radiating  surface,  increasing  the  depth  of  the  magnet  wind- 
ing and  hence  increasing  the  length  of  mean  turn  of  the  magnet 
coil  somewhat  at  the  same  time.  It  also  slightly  decreases 
the  ampere-turns  for  the  magnetic  circuit.  If  we  use  a  large 
pole  pitch,  giving  plenty  of  space  between  the  poles,  together 
with  a  short  armature  and  high  peripheral  speed,  we  can  easily 
avoid  the  increase  in  temperature  due  to  decrease  in  radiating 
surface.  So  that  the  limiting  factor  in  reducing  the  length  of 


FIG. 


the  pole  core  becomes  the  additional  weight  of  copper  due  to  the 
increased  length  of  mean  turn  on  the  magnet  winding,  caused 
by  the  extra  depth  of  the  winding.  With  good  design  we  can 
usually  reduce  the  length  of  pole  core  to  about  one  inch  for 
every  1500  ampere  turns  required  on  full  load;  so  that  our 
leakage  coefficient  will  generally  not  exceed  1.25,  which  is  not 
excessive. 

To  show  the  effect  of  these  various  points  on  the  design  of  a 
machine,  let  us  take  a  definite  example. 


COMMERCIAL   ALTERNATOR   DESIGN 


17 


Output  750  K.W.  60  cycles,  100  R.P.M.,  72  poles,  2200  volt. 
Specification  to  be: 

Electrical  efficiency  at  full  load,  95% 

Regulation  7%  for  P  F  =  1 

16%  for  P  F  =  0.8 
2570  P  F  =  0 

Temperature  rise  on  full  load  PF  ••=  1,  Armature  30°  C., 
Magnets,  20°  C.  This  low  magnet  temperature  being  adopted 
so  that  the  temperature  will  not  become  excessive  with  the 
increased  losses  which  will  result  when  operating  on  loads  of 
low  power-factor. 

A  is  a  machine  which  has  a  pole  pitch  and  diameter  large 
enough  to  use  round  poles,  and  has  saturated  pole  cores.  It 
has  a  strong  armature  and  strong  magnets. 

B  has  a  smaller  pole  pitch,  so  that  it  is  a  longer  machine, 
and  has  unsaturated  fields.  It  has  a  weak  armature  and  field, 
and  the  magnet  winding  is  crowded. 


A. 

B. 

C. 

Internal  diameter  of  armature  
Length  of  armature  core  

207* 
6f 

161" 
13i" 

207" 
61" 

Pole  pitch  

9* 

7" 

9* 

Air  gap                            

5/16" 

1/4" 

5/16" 

Peripheral  speed,  feet  per  min  
Slots  per  pole                    

5400 
6 

4200 
6 

5400 
6 

Turns  per  coil                .        

5 

3 

4 

Magnet  core  section  .             

round 

rectangular 

round 

Induction  in  magnet  core,  per  sq  .  in. 
Regulation  P  F  —    I 

110000 

7% 

9.5000 
6  8% 

110000 
5  6% 

P  F  =  .8 

15  5% 

16% 

10% 

PF  =   0 

24% 

25% 

16% 

Losses  magnet  C2R  watts 
Armature  C2R.                 " 

12500 
10700 

8500 
7250 

17000 
12000 

Iron  loss                             " 

15200 

24000 

19300 

Efficiency 

95  1% 

95  0% 

94  0% 

Temperature  rise  of  armature    .... 
magnets  .  .           . 

22°  C. 
15°  C. 

30°  C. 
16°  C 

26°  C. 
26°  C 

Weight  magnet  copper  .  .           Ibs. 

1800 

:i.s(  H  » 

2300 

"      poles  ' 
"      wheel.  . 

1420 
14000 

4500 
15000 

1780 
16000 

"      armature  copper  .             ' 

1425 

1200 

810 

"              "        laminations  .   ' 
frame  ' 
Cost  of  above  material  

5500 
22000 
$1,645 

7500 
18000 
$2,150 

6600 
22000 
$1,710 

18  COMMERCIAL   ALTERNATOR   DESIGN 

So  on  the  principal  items  that  enter  into  the  cost  of  material, 
the  saving  is  about  25  per  cent,  due  to  the  use  of  strong  armature 
and  magnets,  large  pole  pitch,  and  short  saturated  magnet  cores. 
Probably  the  saving  of  the  cost  of  the  complete  machine  would 
be  about  20  or  25  per  cent.  The  designs  of  these  two  machines 
are  a  little  exaggerated,  but  they  show  very  well  the  saving  in 
cost  that  can  be  made. 

C  is  the  same  machine  as  A,  but  designed  with  a  weaker 
armature,  so  as  to  have  better  regulation,  especially  on  low 
power-factors,  at  the  expense  of  a  lower  efficiency.  The  cost  is 
about  the  same. 

The  chief  points  for  a  cheap  design  for  an  alternator  with 
good  voltage  regulation  on  inductive  loads  are  strong,  saturated 
magnets,  a  reasonably  large  pole  pitch,  and  as  large  an  air-gap 
as  can  be  used  without  excessive  leakage.  In  machines  of  small 
output  with  a  large  number  of  poles,  it  is  impossible  to  get  a 
really  economical  design.  The  diameter  is  decided  by  the  number 
of  poles  and  little  is  gained  by  making  the  machine  less  than 
5  or  6  inches  long,  so  the  cost  is  not  reduced  very  much  with  the 
output.  Generally  speaking,  if  the  output  of  the  machine  is 
less  than  10  K.W.  per  pole,  the  design  is  unnecessarily  expensive, 
while  machines  in  which  the  length  of  the  armature  is  about 
equal  to  the  pole  pitch  are  usually  the  most  economical.  It  is 
for  this  reason  that  60  cycle  alternators  for  small  outputs,  and 
120  or  133  cycle  alternators  of  all  outputs  are  usually  made 
belt-driven;  the  saving  in  cost  by  doing  this  often  being  50 
per  cent.  In  continental  Europe,  where  50  cycles  is  the'  usual 
practice,  belt-driven  alternators  have  never  met  with  much 
favor;  the  universal  custom  being  to  direct-connect  the  alterna- 
tor to  a  low-speed  engine.  The  result  of  this  has  been  that  the 
fly-wheel  type  of  alternator  has  practically  become  standard 
for  this  work,  and  the  poles  of  the  alternator  are  simply  bolted 
to  the  rim  of  the  fly-wheel.  This  type  allows  considerable 
saving  in  cost  in  small  50  or  60  cycle  machines  and  possesses  so 
many  other  advantages  that  it  is  being  introduced  into  this 
country. 


COMMERCIAL    ALTERNATOR    DESIGN  19 

Steam  Turbine  Driven  Alternators. — Alternators  for  direct 
connection  to  steam  turbines  have  lately  come  into  prominence; 
the  chief  consideration  in  these  machines  being,  of  course,  the 
high  speed  at  which  they  operate.  In  order  to  prevent  the 
length  becoming  excessive,  the  diameter  of  a  turbo  alternator 
is  made  as  large  as  possible,  and  peripheral  speeds  of  from 
12,000  to  20,000  feet  per  minute  are  adopted.  The  mechanical 
stresses  in  the  metallic  parts  of  the  magnets  are  high,  but  a  con- 
servative factor  of  safety  can  bo  maintained  if  high-grade 
materials  are  employed.  The  greatest  difficulty  consists  in 
arranging  the  mechanical  stresses  on  the  insulation  of  the  rotor 
in  such  a  way  that  the  insulation  is  not  damaged.  If  this  is 
not  done,  and  if  the  winding  and  constituent  parts  of  the  rotor 
are  not  firmly  fixed  so  that  relative  motion  cannot  take  place 
under  the  influence  of  the  centrifugal  forces,  continual  trouble 
will  result  due  to  changing  of  balance. 

The  electrical  design  of  a  turbo  alternator  is  much  the  same 
as  that  of  a  belt-driven  machine,  except  that  the  speed  being 
very  high  the  efficiency  is  good ;  so  that  the  magnetic  and  current 
densities  in  the  armature  are  limited  only  by  magnetic  saturation 
and  the  difficulty  of  dissipating  the  heat  in  the  extremely  long 
armatures  used  on  these  machines.  The  polo  pitch  and  the 
ampere  turns  on  the  armature  being  large,  the  magnets  are  of 
necessity  very  strong  and  the  air  gap  large,  so  that  the  question 
of  magnetic  leakage  becomes  important.  In  turbo  alternators 
just  as  in  low-speed  machines,  saturated  magnets  with  armature 
and  magnets  as  strong  as  can  conveniently  be  adopted,  result 
in  economical  designs;  but  as  the  mechanical  conditions  are  so 
much  more  severe,  good  mechanical  design  has  a  more  important 
influence  on  the  cost  than  is  the  case  in  a  slow-speed  machine. 

General  Comparison. — When  commencing  to  work  out  a 
machine  an  experienced  designer  can  usually  estimate  the  rnost 
economical  diameter  to  adopt;  and  he  knows  from  experience 
approximately  the  number  of  ampere-turns  he  can  take  per 
inch  periphery  on  the  armature  for  an  alternator  of  given  pole 
pitch  and  type.  This  decides  the  number  of  turns  on  the 


COMMERCIAL   ALTERNATOR    DESIGN 


FIG.  8.— Armature  Winding  of  275  K.W.,  600  R.P.M.,  3  Phase,  60 
Cycle  Generator. 


COMMERCIAL    ALTERNATOR    DESIGN  21 

armature  and  the  ampere-turns  on  the  magnets.  He  then 
completes  the  first  rough  design,  and  working  out  the  perform- 
ance curves,  he  can  usually  see  very  quickly  in  what  way  to 
modify  it  so  as  to  obtain  the  best  design  possible  under  the 
circumstances.  The  speed  and  frequency  are  the  chief  factors 
in  deciding  the  design  of  a  machine,  but  the  voltage,  the  con- 
ditions of  operation,  the  equipment  of  the  factory  in  which  it  is 
to  be  built,  the  facilities  for  obtaining  castings  and  for  shipping 
the  completed  machine,  all  are  points  which  affect  the  design 
and  have  to  be  considered  by  the  practical  designer,  since  the 
prime  object  in  a  commercial  design  is  rather  to  make  profits  for 
the  manufacturing  company  than  to  produce  the  most  perfect 
machine.  The  points  which  have  to  be  taken  into  consideration 
arc  so  numerous  and  varied  that  it  is  impossible  to  give  general 
rules  for  practical  design.  All  that  can  be  done  is  to  give  general 
directions  and  then  it  is  a  question  of  ability  and  experience 
until  the  engineer  can  produce  the  best  results. 

Neglecting  for  a  moment  the  designs  of  Mr.  C.  E.  L.  Brown, 
the  greatest  change  in  the  design  of  alternators  in  the  last  ten 
years  is  the  improved  ventilation  and  the  increased  magnet 
strength.  In  1893  we  were  working  with  air-gap  densities  of 
25,000  to  30,000  and  magnets  with  3,500  to  4,000  ampere-turns 
per  pole  on  full  load,  while  to-day  we  have  air-gap  densities  of 
60,000  to  70,000  and  magnets  giving  anything  up  to  20,000 
ampere-turns  per  pole  on  full  load,  for  ordinary  belt-driven  or 
engine-type  alternators,  and  up  to  double  this  on  steam-turbine- 
driven  machines.  The  change  has  been  made  so  gradually  that 
it  has  been  hardly  noticeable,  but  the  effect  can  be  seen  if  I  give 
the  dimensions  on  two  machines  designed  and  tested,  one  in 
1894  and  the  other  in  1903,  this  latter  machine  being  shown 
in  Fig.  8.  Both  the  designs  are  typical  of  the  condition  of  the 
alternator  design  at  those  dates. 


COMMERCIAL   ALTERNATOR    DESIGN 


1894 

1903 

Output 

(  70  K.W.  3  Phase 

}  275  K  W  3  Phase 

Speed  

(  50  K.W.  Single  Ph. 
600  R.P.M. 

600  R  P  M 

Cycles  

60 

60 

Type  of  magnets    

Lauffen  Type. 

Standard  Revolving 

Internal  diameter  armature  .... 
Length  of  armature  laminations 
Armature  ampere-turns 

37" 

ior 

1380 

Field  Type. 
38" 
10 
1050 

Ampere-turns  on  magnets    
Full  load  efficiency,  per  cent  
Regulation  P  F  =  1 

4700 
90 

11% 

7500 
94 

5  5% 

"          P  F  =  .8 

Would  not  give  volts 

14% 

Full  load  temperature  rise  of  ar- 
mature      .  .  . 

31°  C 

23°  C. 

Total  weight  of  copper,    Ibs. 

730 

680 

Total  weight  of  machine    "   .... 
Total  cost  of  material 

7400 
$455 

11000 
$490 

(Two  bearing  machine  and  15c. 
copper) 

The  output  has  been  increased  about  four  times  and  we  have 
a  much  better  machine  as  regards  performance,  while  the  cost 
is  about  the  same.  The  older  machine  having  such  a  low  out- 
put needs  a  large  amount  of  unnecessary  material,  to  reduce 
the  losses  so  as  to  give  a  reasonable  efficiency.  While  in  the 
larger  output  machine  we  can  afford  a  considerably  higher 
loss  without  reducing  the  efficiency,  so  that  the  weight  of  mate- 
rial is  little  more;  and  the  better  mechanical  design  has  made 
the  cost  of  material  in  the  two  machines  about  the  same.  Speak- 
ing generally,  the  whole  result  has  been  accomplished  by  using 
stronger  magnets  and  higher  densities  throughout,  which  are 
possible  on  account  of  the  improved  ventilation. 

Alternating  current  design  has  rather  stagnated  of  late  on 
account  of  the  limited  competition,  and  most  of  the  recent 
developments  have  come  from  the  other  side  of  the  water.  But 
I  think  it  has  been  shown  that,  from  the  point  of  view  of  dollars 
and  cents,  it  is  certainly  worth  while  to  spend  time  and  ability 
somewhat  lavishly  in  designing  alternators. 


[Paper  presented  before  the  Northwestern 
Electrical  Association,  June  10,  1904.] 


DOUBLE-CURRENT  GENERATORS  IN  THEIR  CONNEC- 
TION WITH  DOUBLE-CURRENT  SUPPLY 


THE  relative  advantages  of  direct  and  alternating  current 
supply  are  now  tolerably  well  recognized.  The  great  advantage 
of  alternating  current  is  the  ease  with  which  high  voltages  can  be 
handled,  and  the  facility  with  which  the  voltages  can  be  trans- 
formed by  means  of  stationary  transformers;  while  the  disad- 
vantage is  that  in  the  present  state  of  the  art  it  is  unsatisfactory 
for  street  railway  work,  and  also  to  a  certain  extent  for  elevator 
or  variable  speed  motors.  With  direct  current,  exactly  the 
reverse  is  the  case;  it  is  unsuitable  for  high  voltage  work,  but 
gives  good  results  in  all  classes  of  motor  work.  The  obvious 
result  of  this  has  been  the  adoption  of  double-current  supply 
in  situations  where  both  these  advantages  and  disadvantages 
are  important.  So  that  usually  in  small  towns  alternating 
current  is  supplied  for  lighting  and  direct  current  for  traction 
work  while  in  larger  towns  direct  current  is  supplied  for  the 
down- town  districts,  where  the  motor  load  is  important,  and 
alternating  current  for  up-town  districts,  where  the  load  is 
almost  entirely  a  lighting  one.  Thus  we  very  often  have  both 
alternating  and  direct  current  supply  from  the  same  power 
station. 

The  question  of  double-current  supply  from  one  station  is 
usually  settled  by  the  installation  of  both  alternating  and  direct 
current  sets,  each  set  generally  having  its  own  engine.  This 
solution  is  hardly  regarded  as  satisfactory  because  the  motor 

23 


24         DOUBLE-CURRENT  GENERATORS 

load,  having  its  maximum  during  the  day,  and  the  lighting  load 
its  maximum  in  the  evening,  the  result  is  that  the  alternating 
current  sets  are  idle  during  the  day  and  the  direct  current  sets 
are  idle  in  the  evening,  so  that  we  have  only  about  half  the  plant 
in  use  at  one  time.  It  is  obvious  that  a  saving  in  first  cost  and 
in  operating  expense  will  be  made  if  the  two  systems  are  tied 
together  in  some  way,  so  that  they  can  help  out  one  another  at 
times  of  heavy  load.  This  can  be  done  by  having  both  an 
alternator  and  a  direct-current  generator  coupled  to  one  engine, 
or  by  having  double-current  generators,  or  by  tying  the  supply 
circuits  together  by  rotary  converters  and  motor-generator  sets. 
A  modification  of  the  latter  method  which  has  recently  be- 
come popular  consists  in  installing  alternating  current  gen- 
erators only,  and  providing  rotary  converters  to  transform  a 
portion  of  the  power  to  direct  current. 

From  the  point  of  view  of  the  station  engineer  the  double- 
current  generator  should  be  the  best  solution.  The  efficiency 
is  higher  than  when  rotary  converters  or  motor  generator  sets 
are  used,  and  it  ought  to  be  considerably  cheaper  than  either 
of  the  other  methods.  The  objection  to  it  is  that  the  voltage 
on  the  alternating  current  side  of  the  generator  bears  a  definite 
ratio  to  that  on  the  direct-current  side,  so  that  one  cannot  be 
varied  without  the  other;  while  the  variation  of  the  load  on  one 
side  affects  to  a  slight  extent  the  voltage  on  the  other.  The 
relative  importance  of  these  objections  has,  of  course,  to  be 
decided  in  each  individual  case.  From  the  manufacturer's 
point  of  view  the  objection  to  double-current  generators  is  that 
they  arc  special  machines,  and  usually  require  new  designs  and 
special  patterns  or  tools. 

Of  course,  we  can  take  any  direct-current  generator,  provide 
it  with  collector  rings,  and  use  it  to  supply  alternating  current; 
but  the  difficulty  is  usually  that  the  frequency  is  unsuitable. 
An  alternator  can  be  built  for  any  commercial  frequency,  speed, 
or  voltage,  without  any  serious  difficulty;  but  for  a  direct-current 
generator,  given  the  speed,  voltage,  and  output,  the  question 
of  commutation  and  cost  decide  within  narrow  limits  the  number 


DOUBLE-CURRENT   GENERATORS 


of  poles,  and  hence  indirectly  the  frequency.  If  it  is  necessary 
to  change  this  number  of  poles  considerably  in  order  to  obtain 
a  special  frequency,  there  is  often  difficulty  with  the  design,  which 
results  in  increased  cost.  So  if  some  latitude  can  be  allowed  in 
selecting  the  frequency  and  speed  for  double-current  generators, 
it  is  advisable  to  choose  them  so  that,  if  possible,  the  generator 
does  not  differ  very  much  from  some  standard  direct-current 
machine. 

The  following  table  gives  the  frequency  of  the  alternating 
current  that  could  be  obtained  from  standard  direct-current 
generators.  The  number  of  poles  and  the  speeds  of  the  machines 
are  taken  from  those  of  the  National  Electric  Company,  but 
there  is  little  difference  in  these  respects  between  the  machines 
of  the  various  manufacturers,  so  that  they  can  be  regarded  as 
applying  approximately  to  all  standard  makes  of  generators. 


ENGINE-DRIVEN. 

BELT-DRIVEN. 

STEAM   Ti  -RHINE 
DRIVEN. 

250  Volt. 

500  Volt. 

250  Volt. 

500  Volt. 

2.50  Volt. 

500  Volt. 

25 

15 

15 

40 

40 

50 

14 

14 

33 

33 

90                 90 

100 

14 

13 

35 

35 

70                70 

250 

14 

12 

27 

23 

60                60 

500 

12 

10 

31 

25 

.    75 

50 

750 

14 

10 

70 

1000 

15 

13 

1500 

20 

16 

2500 

22 

18 

Considering  as  standard  frequencies  for  double-current 
generators  25,  40,  and  60,  it  is  evident  that  some  of  these  stand- 
ard direct-current  machines  could  be  very  conveniently  used  as 
double-current  generators  with  only  a  slight  change  in  speed.  A 
standard  2,500  K.W.,  250  volt  engine  type  machine  would  make 
a  good  25  cycle  double-current  generator,  the  only  changes  nec- 
essary being  to  provide  collector-rings,  and  to  increase  the  air 
gap  and  put  more  copper  on  the  field  magnets  to  make  the 
regulation  satisfactory  when  operating  as  an  alternating-current 


26  DOUBLE-CURRENT   GENERATORS 

generator.  These  changes  would  not  increase  the  cost  of  the 
generator  more  than  20  per  cent.  On  the  other  hand,  if  we  took 
a  500  K.W.,  500  volt  engine  type  machine,  very  radical  changes 
would  be  required  to  make  this  into  a  25  cycle  double-current 
generator,  as  it  would  be  necessary  to  increase  the  number  of 
poles  from  10  to  24  or  30,  which  would  practically  double  the 
cost  of  the  machine.  But  if  we  make  this  500  K.W.  machine 
belt-driven  instead  of  -direct-connected  to  a  slow-speed  engine, 
it  is  evident  from  the  table  of  frequencies  that  a  standard 
machine  would  be  satisfactory  for  25  cycles. 

Twenty-five  or  forty  cycle  machines  are  not  in  any  way 
difficult  to  build — at  the  most  it  is  a  question  of  special  designs 
and  patterns.  But  with  60  cycle  generators  we  begin  to  have 
difficulties  with  the  commutator  on  account  of  the  high  periph- 
eral speed.  Sixty  cycle,  600  volt,  double-current  generators 
and  rotary  converters  can  undoubtedly  be  built,  but  at  the 
present  date  they  are  not  such  reliable  machines  as  those  for 
lower  frequencies,  and  there  is  no  brush  gear  now  on  the  market 
which  is  quite  satisfactory  for  the  peripheral  speeds  necessary 
in  a  60  cycle,  600  volt  machine. 

The  higher  the  speed  of  a  standard  direct-current  machine 
for  a  given  output,  the  higher  the  frequency;  thus  we  would 
expect  that  the  higher  the  frequency  of  a  double-current  genera- 
tor the  higher  the  speed  at  which  it  should  operate;  and  it 
appears  from  the  table  that  the  most  satisfactory  60  cycle 
double-current  generators  will  be  those  driven  by  steam 
turbines.  Direct  current  generators  for  direct  connection  to 
steam  turbines  and  suitable  for  operation  under  American  con- 
ditions, are  not  at  present  on  the  market,  but  in  all  probability 
they  will  be  shortly;  and  it  appears  probable  that  this  type  of 
machine  will  solve  the  problem  of  60  cycle  double-current 
generators. 

Generally  speaking  then,  25  cycle  double-current  generators, 
if  of  large  size,  can  be  direct-connected  to  a  steam  engine,  while 
for  smaller  units  than  500  K.W.,  they  are  better  belt-driven. 
Forty  cycle  machines  should  always  be  belt-driven  if  the  cost 


DOUBLE-CURRENT   GENERATORS  27 

is  to  be  reasonable,  while  for  60  cycle  double-current  generators 
apparently  the  only  reasonable  solution  is  to  have  steam  turbine 
driven  sets.  Of  course,  double-current  generators  can  be  made 
for  any  frequency  or  voltage  up  to  60  cycle,  600  volts,  and  at 
any  speed  desired,  it  is  only  a  question  of  cost;  but  to  obtain  a 
reasonable  price  or  delivery,  and  to  have  a  unit  which  will  at 
some  time  in  the  future  have  a  second-hand  value  better  than 
scrap,  it  would  be  advisable  to  consider  the  above  table  of 
frequencies  and  outputs  when  laying  out  a  station  for  double- 
current  supply  with  double-current  generators. 

NOTE  [Dec.,  1910] 

In  the  past  three  years  great  improvements  have  been  made 
in  the  construction  of  commutators  and  brush  gear  for  operation 
at  high  speeds;  this  work  having  been  done  in  connection  with 
development  of  60  cycle  rotaries  and  direct-current  turbo 
generators,  which  should  be  as  reliable  in  operation  as  the  cor- 
responding 25  cycle  and  slow-speed  units.  The  result  of  this 
work  has  been  to  revolutionize  the  methods  of  constructing  high 
speed  commutators  and  brush  gears,  so  that  at  the  present  time 
commutators  are  being  built  to  operate  perfectly,  with  reason- 
able attention,  at  the  speeds  required  by  60  cycle  rotaries  and 
direct-current  turbo  generators.  This  being  the  case,  the  state- 
ment above,  in  regard  to  the  reliability  of  60  cycle  600  volt 
double-current  generators  and  rotary  converters  should  be 
modified  accordingly. 


[Presented  before  the  American  Institute  of 
Electrical  Engineers,  May  17, 1904.] 


PREDETERMINATION   OF   SPARKING    IN   DIRECT 
CURRENT   MACHINES 


SPEAKING  generally,  dynamo  design  did  not  become  an  art 
until  after  the  old  two  pole  smooth-core  Siemens  and  Edison 
machines  came  into  extensive  use  for  electric  lighting.  The 
original  design  of  these  machines  was  more  or  less  guesswork ;  but 
after  a  few  machines  had  been  made  to  operate  satisfactorily,  the 
engineer  was  able  to  lay  out  a  complete  line  of  machines,  design- 
ing them  partly  by  his  engineering  intuition,  and  partly  by  some 
empirical  rules,  which  he  decided  on  as  he  built  successive 
machines.  The  armatures  were  designed  more  from  a  mechani- 
cal than  from  an  electrical  standpoint,  their  length  being  limited 
by  the  stiffness  of  the  shaft  rather  than  by  questions  of  com- 
mutation; while  they  were  unvcntilated,  and  in  consequence 
the  output  was  limited  by  heating.  The  armatures  being  of  the 
smooth-core  type,  the  self-induction  of  the  armature  coils  was 
usually  so  small,  even  with  the  length  of  armatures  in  general 
use,  that  it  was  unnecessary  to  consider  it  in  connection  with  the 
tendency  of  the  machine  to  spark.  It  was,  however,  generally 
recognized  that  if  the  magnets  were  too  weak  the  machine  was 
liable  to  spark,  so  the  length  of  the  air-gap  was  usually  deter- 
mined by  some  empirical  rule  obtained  by  experiment.  When 
slotted  armatures  were  first  adopted  extensively  they  were  de- 
signed along  the  same  lines  as  smooth-core  armatures.  They 
were  so  badly  ventilated  that  the  output  was  limited  by  heating 
to  about  one-half  that  of  a  modern  armature;  but  in  spite  of 

29 


30          SPARKING  IN  DIRECT  CURRENT  MACHINES 

this  it  was  found  necessary  to  use  carbon  brushes  to  obtain  good 
commutation.  To  economize  in  tools  several  different  lengths 
of  armatures  were  frequently  built  on  the  same  diameter,  while 
to  economize  space,  the  armatures  were  often  built  smaller  in 
diameter  and  longer  than  they  otherwise  would  be.  Experience 
with  these  different  forms  of  armature  made  it  very  evident  that 
a  long  armature  had  a  greater  tendency  to  spark  than  a  short 
one;  and  this  became  especially  noticeable  as  the  ventilating 
was  improved  and  the  output  correspondingly  increased  on 
account  of  the  cooler  operation. 

Previous  to  this  a  great  deal  had  been  written  on  the  theory 
of  commutation  in  dynamos,  but  had  been  ignored  by  the  prac- 
tical designers,  who  had  more  faith  in  experimental  results. 
But  this  bad  behavior  of  long  armatures  as  regards  sparking, 
called  attention  to  the  theoretical  work,  and  designers  began  to 
consider  whether  or  not  the  self-induction  of  the  armature  coils 
did  not,  after  all,  decide  the  amount  of  current  the  machine 
would  commutate  without  sparking.  In  the  first  attempts  to 
take  into  account  the  self-induction  of  the  commutated  coil, 
the  self-induction  of  a  one-turn  coil  was  considered  as  being 
simply  proportional  to  the  length  of  the  armature  core;  that 
is,  the  shape  or  size  of  the  slot,  the  number  of  coils  per  slot,  and 
•the  self-induction  of  the  end  connections,  were  all  neglected. 
'This  gave  a  very  simple  formula  for  the  self-induction : 
I  L  =  ln\ 

Where  I  =  length  of  armature 

n  =  number  of  turns  per  coil, 

And  the  self-induction  E.M.F.  of  commutation  (the  reactance 
voltage  as  it  was  called),  which  is  an  estimate  of  the  difficulty  of 
commutating  the  current,  was  given  by 

E  =  In2  if 

i  being  the  current  per  coil  and  /  the  frequency  of  commuta- 
tion. 

This  formula  gave  satisfactory  results  when  applied  to  ma- 
chines designed  along  the  same  general  lines.  The  allowable 
value  of  the  reactance  voltage  could  be  obtained  from  experiment 


SPARKING  IN   DIRECT  CURRENT .  MACHINES         31 

on  one  machine,  and  used  in  the  design  of  other  similar  machines. 
But,  if  applied  to  machines  which  were  designed  differently,  the 
formula  showed  wide  discrepancies;  so  it  soon  became  recognized 
that  the  formula  was  at  best  only  a  rough  approximation. 

Early  slotted  machines  were  designed  with  one  coil  per  slot; 
two  coils  per  slot  obviously  saved  insulation  space  and  were  soon 
tried,  but  it  was  found  that  generally  if  this  were  done  every 
other  bar  on  the  commutator  became  badly  marked.  As  it 
was  imperative  to  save  space  in  car  motors,  three  coils  per  slot 
were  adopted,  and  in  extreme  cases  four,  or  even  five  coils  per 
slot  were  used.  It  was  generally  found,  however,  that  whenever 
more  than  one  coil  per  slot  was  used  some  of  the  commutator- 
bars  were  marked,  and  that  it  was  possible  to  count  the  number 
of  coils  per  slot  by  the  recurrence  of  the  marking  on  the  com- 
mutator. This  marking  was  attributed  to  the  inequality  caused 
by  using  a  small  number  of  slots,  and  so  the  general  rules  were 
adopted  to  use  as  many  slots  as  possible  and  to  make  small 
machines  with  one  coil  per  slot  and  large  machines  with  only 
two  coils  per  slot. 

It  was  also  noticed  that  the  dead  coils  necessary  in  certain 
multipolar  wave-windings  often  caused  some  of  the  commutator 
bars  to  be  marked.  This  was  naturally  attributed  to  the  dis- 
symmetry produced  in  the  winding,  and  it  became  generally 
recognized  that  anything  tending  to  produce  inequality  in  the 
commutation  conditions,  such  as  few  slots,  or  many  coils  per 
slot,  or  dead  coils,  tended  to  make  perfect  commutation  more 
difficult. 

With  increased  competition  came  the  necessity  of  cheapening 
the  cost  of  building  these  machines;  designers  then  returned  to 
the  construction  of  several  coils  per  slot.  In  reducing  the 
amount  of  copper  on  the  armatures  to  save  in  the  cost  of  ma- 
terial, it  naturally  happened  that  shallow  slots  were  used.  And 
it  was  found  that  with  these  wide  and  shallow  slots  it  was  possible 
to  obtain  good  commutation  with  several  coils  per  slot,  under 
conditions  where  it  would  be  quite  impossible  with  the  old  deep 
and  narrow  slots.  Obviously  this  was  due  to  the  lesser  self- 


32          SPARKING  IN  DIRECT   CURRENT   MACHINES 

induction  of  a  wide  slot  compared  to  a  narrow  one,  and  it  was 
soon  acknowledged  that  the  shape  of  the  slot  should  be  con- 
sidered in  calculating  the  self-induction  of  the  commutated  coil. 

When  designing  an  armature  for  small  self-induction  it  would 
be  natural  to  make  it  large  in  diameter  and  short  in  length ;  that 
is,  with  a  large  pole-pitch.  But  in  carrying  this  to  an  extreme 
it  was  found  that  it  did  not  give  the  good  results  expected.  It 
was  suggested  that  this  result  was  due  to  the  fact  that  the  self- 
induction  of  the  end  connections  had  been  neglected,  and  that  in 
armatures  with  large  pole-pitch  and  short  length  of  core,  the 
self-induction  of  the  end  connections  was  comparable  with  that 
of  the  conductor  embedded  in  the  slots. 

In  the  light  of  these  experiences  it  is  evident  that  the  design 
of  a  direct-current  machine  in  regard  to  sparking  is  a  compromise 
between  a  number  of  conflicting  conditions.  It  is  not  possible 
to  obtain  a  formula  which  will  give  a  strict  measure  of  the  corn- 
mutating  qualities  of  all  machines;  but  by  taking  into  considera- 
tion the  more  important  conditions  which  affect  the  sparking,  it 
is  possible  to  obtain  one  which  will  give  fairly  accurate  results 
when  applied  to  machines  similarly  designed,  and  which  will 
give  some  idea  of  the  tendency  to  spark  when  applied  to  machines 
of  widely  different  design.  Such  a  formula,  when  it  has  been 
applied  to  numerous  machines  of  different  types,  so  that  the 
allowable  values  for  the  sparking  constant  have  been  determined, 
can  be  taken  as  a  fair  working  formula,  and  can  be  placed  in  the 
same  category  as  empirical  formulae  for  determining  the  regu- 
lation of  alternators.  Such  formula)  are  not  intended  to  reduce 
designing  to  mere  slide-rule  work,  but  are  intended  simply  to 
give  an  idea  as  to  the  experimental  results  to  be  expected  from 
an  individual  design. 

As  outlined  above,  the  most  important  conditions  to  be  taken 
into  consideration  are  the  self-induction  electromotive  force  of 
the  commutated  coil,  and  the  inequalities  introduced  by  the 
conditions  of  commutation. 

Electromotive  Force  Due  to  Self-induction.  This  is  given  by 
the  formula :  E  =  Self-induction  of  one  coil  X  number  of  coils 


SPARKING   IN  DIRECT  CURRENT  MACHINES         33 

commutated  in  series  X  current  in  coil  X  frequency  of  commu- 
tation. 

The  self-induction  of  one  coil  =  (self-induction  of  one  con- 
ductor embedded  in  the  slot  +  self-induction  of  one  end  connec- 
tion) X  (number  of  turns  per  coil)2. 

The  self-induction  of  one  conductor  embedded  in  the  slot  =  Ik. 
Where  I  is  the  length  of  the  core  and  A;  is  a  constant  depending 
on  the  dimensions  of  the  slot. 

By  determining  the  self-induction  of  a  large  number  of  slots 
we  find  that  this  constant  k  can,  with  sufficient  accuracy,  be 
taken  as  a  function  of  the  ratio  r,  where 

Width  of  slot. 
Depth  of  slot. 

A  curve  can  be  plotted  connecting  r  and  k,  determined  ex- 
perimentally from  tests  on  a  number  of  armatures;  such  a  curve 
is  shown  in  Fig.  9. 

The  self-induction  of  the  end  connections  can  be  taken  as  = 
length  of  end  connections  X  constant  c/.  And  as  the  length  of 
end  connection  is  approximately  proportional  to  the  pole  pitch, 
the  self-induction  of  two  end  connections  can,  with  sufficient 
accuracy,  be  written  =  p  c. 

Hence  the  self-induction  of  one  coil  =  n2  (I  k  -f  p  c). 

The  number  of  coils  commutated  in  series,  N,  is,  of  course, 
one  in  a  parallel  or  lap- wound  armature,  and  equal  to  the  number 
of  pairs  of  poles  in  a  series  or  wave-wound  armature. 

The  current  per  coil  i  in  a  two-circuit  series  or  wave-wound 
armature  is  equal  to  one-half  the  total  current  in  the  machine, 
while  in  a  parallel  or  lap-wound  armature  it  equals  the  total 
current  divided  by  the  number  of  poles. 

The  self-induction  pressure  of  the  commutated  coil  is  then 
given  by 

V  =  n2  (Ik  +  p  c)  N  if 

Where  /  is  frequency  of  commutation,  and  is  equal  to  the 
number  of  commutator  bars  X  speed  in  rev.  per  min. 

3 


34 


SPARKING  IN  DIRECT  CURRENT  MACHINES 


The  width  of  the  brush  is  neglected  in  calculating  the  fre- 
quency of  commutation,  since  it  is  found  by  experiment  that 
within  the  ordinary  limits  of  practice  the  thickness  of  the  brush 
has  little  effect  on  the  operation  of  a  machine,  unless  the  current 
density  is  excessive.  The  probable  explanation  of  this  is  that 
a  thicker  brush  gives  more  time  for  commutation  to  take  place; 
but  it  also  means  that  more  coils  are  commutated  at  the  same 


FIG.  9. 

time,  thus  introducing  the  effect  of  mutual  induction.  These 
two  effects  apparently  counterbalance  each  other  to  a  great 
extent. 

Inequalities  Due  to  Conditions  of  Commutation. — These  are 
due  to  the  use  of  few  slots;  more  than  one  coil  per  slot;  and  to 
dead  coils. 

If  there  is  only  one  coil  per  slot  the  use  of  few  slots  does  not 


SPARKING   IN   DIRECT  CURRENT  MACHINES         35 

in  itself  affect  commutation,  unless  the  number  of  slots  is  ex- 
tremely small;  for  though  the  slot  may  move  through  an  ap- 
preciable arc  while  the  coil  is  being  commutated,  the  conditions 
are  exactly  the  same  for  every  coil  when  it  is  commutated. 
So  there  is  no  tendency  to  inequality  in  the  conditions,  and  if 
it  is  possible  to  set  the  brushes  so  that  one  coil  can  be  commutat- 
ed satisfactorily,  then  commutation  will  be  satisfactory  for  all 
the  coils.  But  if  the  number  of  slots  is  extremely  small,  say  less 
than  six  per  hole,  then  the  coil  will  move  in  such  a  widely  varying 
magnetic  field,  and  will  come  so  close  to  the  strong  field  under 
the  pole-tip  while  it  is  being  commutated,  that  the  local  currents 
under  the  brush  are  liable  to  produce  marking  of  the  commuta- 
tor-bars even  if  the  brushes  apparently  do  not  spark.  Of  course 
this  is  only  important  in  very  low  voltage  machines  and  it  is 
unnecessary  to  take  it  into  account  in  any  constant  which  is  to 
be  a  criterion  of  the  tendency  to  spark.  It  is  sufficient  to  say 
that  the  number  of  commutator-segments  in  the  polar-gap, 
that  is,  the  arc  between  the  two  pole-shoes,  must  never  be  less 
than  two  and  should  generally  be  three  or  more. 

With  more  than  one  coil  per  slot  inequalities  are  introduced: 
due  to  the  difference  in  the  value  of  the  self-induction  of  the 
various  coils;  and  due  to  their  commutation  under  different 
conditions. 

The  self-induction  of  all  the  armature  coils  will  be  the  same 
when  there  are  only  two  coils  per  slot,  as  it  is  obvious  that  the 
configuration  of  the  conductors  and  neighboring  iron  is  the  same 
for  both  coils.  But  when  there  are  three  or  more  coils  per  slot 
the  self-induction  of  the  various  coils  will  vary,  as  they  occupy 
different  relative  positions  in  regard  to  the  iron;  the  self-induc- 
tion of  the  center  coil  being  less  than  the  self-induction  of  the 
outer  coil.  Investigating  conditions  at  the  point  of  commuta- 
tion in  a  modern  generator  by  means  of  a  pilot-brush,  it  is 
found  that  commutation  usually  takes  place  at  a  point  where 
there  is  practically  no  resultant  magnetic  field;  that  is,  at  a  point 
where  the  magnetic  field  of  the  armature  just  counterbalances 
the  average  field  due  to  the  magnets.  In  other  words,  there  is 


36 


SPARKING   IN   DIRECT   CURRENT   MACHINES 


resistance  commutation;  the  armature  current  is  commutated 
by  the  varying  resistance  of  the  brush,  rather  than  by  a  reversing 
E.M.F.,  due  to  passing  through  a  magnetic  field.  This  being 
the  case  it  is  only  necessary  to  consider  the  self-induction  of 


FIG.  10. — Showing  Position  of  Armature  at  Beginning  and  End  of 
Commutation  Period. 

those  coils  which  have  the  greatest  self-induction.  If  these  are 
commutated  satisfactorily  by  means  of  the  varying  resistance 
of  the  brushes,  then  the  coils  which  have  a  smaller  self-in- 
duction will  also  be  satisfactorily  commutated.  Hence  the 
variation  in  the  self-induction  of  the  coils  need  not  be  con- 


SPARKING  IN  DIRECT  CURRENT  MACHINES 


37 


sidered,  and  in  the  formula  all  that  it  is  necessary  to  consider  is 
the  self-induction  of  those  coils  which  have  the  greatest  self- 
induction. 

The  chief  inequality  introduced  into  the  commutation  by 
the  adoption  of  more  than  one  coil  per  s'ot  is  due  to  the  various 


FIG.  11. — Armature  with  Two  Coils  per  Slot  Showing  Position  of  Armature 
when  the  Two  Coils  are  being  Commutated. 

coils  in  the  slot  being  commutated  when  they  are  in  different 
magnetic  fields.  This  is  evident  from  Fig.  11,  which  shows  the 
position  of  the  armature  when  the  first  and  the  last  coil  in  the 
slot  are  being  commutated.  If  the  brushes  are  set  so  that  the 
magnetic  field  is  right  for  the  first  coil  it  will  be  incorrect  for  the 


38          SPARKING  IN  DIRECT  CURRENT  MACHINES 

last  one,  and  vice  versa.  So  whenever  the  machine  is  loaded  to 
its  limit  the  commutating  conditions  may  be  so  bad  for  some  of 
the  coils,  that  in  time  some  of  the  commutator-bars  will  become 
pitted  and  the  well-known  regularly  recurring  marking  of  the 
commutator-bars  will  develop. 

The  question  is  how  to  take  this  inequality  into  account  in 
the  sparking  formula.  To  do  this,  we  make  the  assumptions 
that  the  magnetic  field  varies  uniformly  from  the  neutral  point 
to  the  pole-tip,  and  that  in  order  to  obtain  perfect  commutation 


FIG.  12. 

it  is  necessary  to  move  the  brushes  from  a  position  on  the  neutral 
point  at  no  load,  to  a  position  half-way  between  the  neutral 
point  and  the  pole-tip  at  full  load.  Calling  the  distance  be- 
tween the  neutral  point  and  the  pole-tip  2  d,  and  assuming 
the  brushes  fixed  on  the  line  0  P  half-way  between  the  pole-tip 
and  the  neutral  point,  then  if  any  coil  is  commutated  when  it  is 
at  Q  distant  "a"  from  P,  the  machine  will  only  commutate 

perfectly  a  current  corresponding  to  1  -  —  of  full  load. 

a 

Just  what  this  assumption  means  can  be  seen  from  Fig.  13. 


SPARKING   IN  DIRECT  CURRENT  MACHINES 


39 


Abscissae  represent  positions  along  the  polar-gap  corresponding 
to  Fig.  12,  and  ordinates  represent  E.M.F's.  The  line  N  A  gives 
the  E.M.F.  induced  at  various  points  by  the  conductor  moving 
in  the  field  due  to  the  magnets.  C  P,  the  ordinate  of  the  line 
B  B,  gives  the  E.M.F.  necessary  to  reverse  the  full-load  current 
7  in  the  coil.  If  the  coil  is  commutated  at  the  position  Q  instead 
of  at  P  then  the  commutation  conditions  will  be  perfect  only 
DQ 


for  a  current 


Hence  we  assume  that  if  we  have  sev- 


eral coils  per  slot,  and  that  if  in  consequence  of  this  we  have  to 


Q  P  Q' 

Circumference  of  Armature 

FIG.  13. 


commutate  some  of  our  coils  in  a  position  E  Q  and  E'  Q',  then 
the  current  which  the  machine  will  carry  without  sparking  is 

reduced  in  the  ratio  -  — —  that  is 


CP  '  NP' 

It  is  very  easy  to  figure  out  what  this  inequality  amounts  to 
in  any  particular  case.  Take  20  slots  per  pole,  3  coils  per  slot, 
and  pole-face  =  75  per  cent  of  pole-pitch.  There  are  2.5  slots  be- 
tween the  neutral  point  and  the  pole-tip.  Assuming  that  the  con- 
ditions are  perfect  for  the  centre  coil,  the  outer  coils  are  0.33 
slot  pitch  distant  from  this  most  favorable  position.  And  1.25 


40 


SPARKING  IN  DIRECT  CURRENT  MACHINES 


slots  corresponding  to  variation  from  no  load  to  full  load,  hence 

0  33 

an  equality  of   0.33  slot- pitch  gives  an  inequality  factor  -'- — 

1 ._  •  > 

=  0.26;   so  that  the  sparking  constant  should  be  multiplied  by 
the  inequality  factor  1.26. 

Curves  can.  very  easily  be  plotted  for  different  numbers  of 
slots  per  pole  and  coils  per  slot,  in  order  to  facilitate  the  calcu- 
lation of  this  inequality  factor.  Such  curves  are  shown  in  Fig.  15. 
The  assumptions  on  which  this  calculation  is  based  are  to  a 
great  extent  rational,  and  though  we  cannot  pretend  that  the 


FIG.  14. — Armature  with  Twenty  Slots  per  Pole  and  Three  Coils  per  Slot. 

calculation  has  a  rigid  basis,  yet  it  is  probably  as  correct  as  the 
other  sparking  calculations,  and  used  with  discretion  it  gives 
fairly,  reliable  results. 

The  inequality  introduced  by  the  use  of  a  dead  coil  on  the 
armature  is  similar  to  that  due  to  several  coils  per  slot.  The 
dead  coil  produces  a  break  in  the  uniformity  of  the  winding; 
and  if  the  position  of  the  brush  is  correct  for  commutation  of 
the  coil  immediately  on  one  side  of  the  dead  coil  then  it  will  be 
one  segment  out  of  the  correct  position,  for  the  coil  immediately 
on  the  other  side  of  the  dead  coil.  The  inequality  introduced 
can  be  calculated,  and  allowed  for,  in  exactly  the  same  way  as 


SPARKING  IN  DIRECT   CURRENT  MACHINES         41 

we  estimate  the  inequality  due  to  several  coils  per  slot.  Assum- 
ing that  the  brush  is  in  a  mean  position,  then  it  will  be  half 
a  segment  out  of  position  for  the  two  coils  which  are  next 
in  position  to  the  dead  coil.  So  making  the  same  assumptions 
as  before;  if  there  are  n  commutator-segments  per  pole,  and  if 


=4  Coils  i>er  Slot 
8=3  Coils  per  Slot 
C  =  2  Coils  per  Slot 


1.0 

10  20  30 

Slots  per  Pole 
FlG.  15. 

the  pole  face  =  75  per  cent  of  pole-pitch,  then  the  inequality 

Q 

is  equivalent  to  — .      Thus  if  there  are  20  segments  per  pole,  a 

dead  coil  produces  an  equality  equal  to  40  per  cent  of  the  load, 
and  the  inequality  should  be  introduced  into  the  sparking  con- 
stant by  the  factor  1.4.  A  curve  can  readily  be  plotted  between 


42          SPARKING  IN  DIRECT   CURRENT  MACHINES 

the  inequality  factor  and  the  number  of  commutator  segments 
per  pole.     Such  a  curve  is  shown  in  Fig.  17. 

General  Formula. — Combining  all  the  various  factors  which 
affect  sparking  we  get  as  our  complete  formula  for  a  sparking 
constant 

C  =  rf  (Ik  +  pc)NifPQ. 

P  being  the  inequality  factor  resulting  from  a  number  of 


FIG.  16. — Armature  with  Dead  Coils  Showing  Positions  of  Armature  when 
the  Two  Coils  Next  to  Dead  Coil  are  being  Commutated. 

coils  per  slot,  and  Q  the  corresponding  factor  due  to  the  presence 
of  a  dead  coil. 

This  formula  is  not  put  forward  as  being  scientifically  exact, 
but  as  an  empirical  formula  which  has  gradually  been  built  up 


SPARKING  IN   DIRECT   CURRENT   MACHINES          43 

as  the  result  of  experience ,  different  terms  having  been 
added  to  the  formula  from  time  to  time  when  it  was  found 
necessary  to  take  different  conditions  into  account.  As  the 
formula  stands  it  gives  good  results,  when  we  know  the  value 
of  C  which  can  be  allowed  for  the  particular  design  of  machine 
considered. 

The  relative  values  of  C  that  have  been  found  allowable  in 
different  cases  arc  somewhat  as  follows: 

2-pole 20 

4-polc,  scries  two-circuit  winding     35 

f>-pole,      "  .")() 

4-pole,  multiple  wound    30 

6-pole,  "       35 

gradually  increasing  to 

24-pole,  multiple  wound    50 

These  relative  values,  of  course,  only  apply  when  the  machines 
in  each  class  are  designed  with  similar  constants.  That  is,  they 
should  have  approximately  the  same  densities  in  the  teeth,  and 
approximately  the  same  ratio  of  ampere  turns  per  pole  on  the 
armature  (armature1  reaction)  to  the  ampere-turns  required  for 
the  teeth  and  air  gap.  If  these  vary  much  it  is  difficult  to  get 
consistent  results.  The  brush-gear  and  the  current  density  in 
the  brushes  also  play  an  important  part  in  the  sparking.  If  the 
brush-gear  is  weak  mechanically,  or  if  the  commutator  is  in 
bad  condition,  sparking  is  sure  to  take  place;  while  with  the 
average  carbon  brush,  burning  will  usually  take  place  when 
the  current  density  reaches  50  amperes  per  sq.  in.  The  shape 
of  the  pole-tips  has  some  effect  on  the  operation  of  the  machine. 
But  as  long  as  they  are  not  too  close  together,  and  as  long 
as  they  are  shaped  so  that  the  commutation  field  varies 
gradually,  the  exact  shape  need  give  us  no  concern.  The 
density  in  the  armature  core  (behind  the  teeth)  has  also  some 
effect  on  the  allowable  sparking  constant;  and  if  the  core  is 
highly  saturated  a  higher  constant  can  be  used  than  if  it  is 
unsaturated. 


44 


SPARKING  IN   DIRECT   CURRENT   MACHINES 


Assuming  that  all  these  conditions  are  uniform  and  satisfac- 
tory, the  variation  in  the  allowable  value  of  C,  found  in  actual 
practice,  shows  that  the  formula  does  not  take  into  account  all 
the  conditions  that  affect  the  sparking,  so  the  formula  must  be 


1.9 
1.8 

1.7 
1.8 
1..') 
1.4 
1.3 
1.S 
1.1 
1  0 

\ 

V 

\ 

\ 

\ 

\ 

\ 

^ 

\ 

10  20  30 

FIG.  17. — Commutator  Segments  per  Pole-pitch. 

used  with  considerable  discretion.  It  cannot  be  claimed  that 
it  is  in  any  way  accurate,  but  it  can  be  considered  an  empirical 
working  formula,  capable  of  giving  good  results  when  carefully 
used ;  and  as  such  it  is  put  forward. 


[Presented  before  the  American  Institute  of 
Electrical  Engineers,  June  19,  1905.] 


ROTARY    CONVERTERS    AND    MOTOR-GENERATORS 


AT  the  present  time  the  alternating  current  motor  in  a 
motor-generator  set  of  100  K.W.  capacity  or  larger,  is  usually 
a  synchronous  motor;  an  induction  motor  is  seldom  used  for 
this  purpose.  The  reasons  for  this  are,  that  the  lagging  current 
taken  by  an  induction  motor  renders  it  undesirable  at  the  end 
of  a  long  line,  that  from  an  operating  standpoint  the  mechanical 
construction  of  an  induction  motor  makes  it  less  reliable 
than  a  synchronous  motor,  and  that  the  cost  of  an  induction 
motor  has  been  heretofore  appreciably  higher  than  that  of  the 
corresponding  synchronous  motor.  The  usual  objections  to 
the  synchronous  motor — that  it  has  a  low  starting  torque 
and  that  it  requires  external  excitation — do  not  apply  to  the 
case  of  a  synchronous  motor  used  in  a  motor-generator  set,  as 
a  high  starting  torque  is  unnecessary  and  as  there  is  always 
some  way  of  exciting  the  motor  whether  it  is  coupled  to  a  direct- 
current  or  to  an  alternating  current  generator.  It  has  thus 
become  practically  standard  practice  to  use  synchronous  motor- 
generator  sets  in  all  sizes  except  where  the  output  is  too  small  for 
a  standard  synchronous  motor.  This  being  the  case  it  is  only 
necessary  to  consider  synchronous  motor-generator  sets  in 
comparison  with  rotary  converters. 

Motor-generators  and  rotary  converters  can  be  discussed  from 
two  points  of  view;  that  of  the  operating  engineer,  or  that  of  the 
designer  and  manufacturer.  As  the  operating  point  of  view  is 
probably  most  familiar  to  engineers,  that  will  be  considered  first. 

45 


46 


ROTARIES  AND  MOTOR-GENERATORS 


Cost  and  Floor  Space. — The  main  points  that  concern  the 
engineer  when  installing  transforming  machinery  are  the  first 
cost  of  the  machinery,  its  efficiency,  and  its  reliability  and 
flexibility  of  operation.  Incidentally,  the  floor  space  occupied, 
and  sundry  other  things  have  to  be  taken  into  consideration. 
The  cost  of  a  motor-generator  or  rotary  converter,  or  rather 
the  price  at  which  it  is  sold  by  the  manufacturer,  depends  upon 
the  output  and  the  speed,  and  incidentally  upon  the  competition 
among  the  firms  that  are  trying  to  secure  the  business.  The 
choice  of  the  speed  for  either  machine  being  usually  left  to  the 
manufacturer,  is  as  high  as  is  consistent  with  good  mechanical 
and  electrical  design.  The  following  table  gives  speeds  in 
R.P.M.  which  may  be  regarded  as  more  or  less  standard  for  such 
machines  of  different  output,  frequencies,  and  voltages. 

25  CYCLES 


MOTOR-GENERATORS. 

ROTARY  CONVERTERS. 

250  Volts. 

600  Volts. 

250  Volts. 

600  Volts. 

250 

750 

750 

500 

750 

500 

500 

500 

300 

500 

1000 

250 

250 

187 

250 

1500 

214 

214 

150 

214 

2000 

187 

187 

125 

167 

60  CYCLES 


MOTOR-GENERATORS. 

ROTARY  CONVERTERS. 

250  Volts. 

600  Volts. 

250  Volts. 

600  Volts. 

250 

720 

720 

720 

900 

500 

514 

514 

450 

600 

1000 

240 

240 

225 

300 

1500 

189 

189 

189 

240 

2000 

150 

150 

150 

189 

In  comparing  the  cost  of  motor-generators  and  rotaries 
it  may  be  assumed  that  it  will  always  be  necessary  to  use  trans- 
formers with  the  latter  in  order  to  get  the  comparatively  low 


ROTARIES  AND   MOTOR-GENERATORS 


47 


alternating  current  voltage  required.  With  motor-generators, 
on  the  other  hand,  the  motor  can  be  wound  to  take  the  high- 
tension  current  without  the  interposition  of  transformers,  unless 
the  line  voltage  exceeds  15,000  volts.  In  estimating  the  costs 
of  motor-generator  sets  it  is  assumed  that  no  transformers  are 
necessary.  In  general,  any  table  of  relative  costs  of  motor- 
generators  and  rotary  converters  should  be  accepted  with  a 
certain  amount  of  reserve;  as  each  individual  installation  must 
be  considered  by  itself  and  the  costs  of  the  various  items  com- 
pared. The  cost  of  the  switchboard  and  cables  should  also  be 
considered,  and  in  this  respect  the  motor-generator  is  usually 
cheaper  than  the  converter.  The  following  table  gives  the  cost, 

500  K.W.  600  VOLTS,  25  CYCLES 


Rotary  Converter. 

Motor-Generator. 

Cost 

$4500  +  2700  =  $7200 

$9000 

Efficiency  1.25  load  
"          i         « 

91.5 
91  5 

88. 
87  5 

"          0.75    "  
"          005    " 

91.0 
88  5 

85.5 
81  0 

Floor  soace  .  . 

60  +  50  =  110sa.  ft. 

85  so.  ft. 

500  K.W.  600  VOLTS,  60  CYCLES 


Rotary  Generators. 

Motor-Generator. 

Cost  . 

$4700  +  2300  -  $7000 

$8700 

Efficiency  1.25  load  . 

90  5 

88 

"          1 

90  5 

87  5 

"          0.75    "     

89  5 

85  5 

"          0.50    "     ... 

86  5 

81 

Floor  space 

70  +  50—120  sq  ft 

90  sq  f  t 

efficiency,  and  the  floor  space,  required  for  rotary  converters  and 
motor-generators  of  different  outputs.  The  rotaries  are  assumed 
to  operate  in  connection  with  three  single  phase  6,600  volt 
transformers,  and  the  motor-generators  to  operate  on  6,600 


48 


ROTARIES  AND  MOTOR-GENERATORS 


volts  without  transformers.  The  efficiencies  are  the  combined 
efficiencies  of  the  sets;  rotaries  and  transformers  in  the  one  case, 
and  motors  and  generators  in  the  other.  In  the  case  of  rotaries, 
under  the  head  of  cost  and  floor-space,  the  first  figure  refers  to 
the  rotary  and  the  second  to  the  transformers. 

1500  K.W.,  275  VOLTS,  25  CYCLES 


Rotary  Converter. 

Motor-Generator. 

Cost 

$18000  +  6300  —  $24300 

$21000 

Efficiency  1.25  load 

93  5 

90  5 

"           1 

93  5 

90 

"           075    " 

92  5 

88 

"           050    " 

90  5 

85 

Floor  space 

240  -f  125  =  345  sq   ft 

320  sq   ft 

The  above  are  sale  prices  f.o.b.  factory  and  do  not  include 
freight  or  erection  charges.  All  necessary  rheostats  and  shunts 
are  included,  but  no  induction  regulators  for  the  rotaries. 

In  all  three  cases  it  will  be  noticed  that  the  rotary  converter 
and  transformers  are  the  more  efficient,  the  difference  in  efficiency 
being  about  3  per  cent  at  full  load  and  about  6  per  cent  at  half 
load.  The  value  of  this  difference  in  efficiency  has  to  be  decided 
in  each  case  by  the  cost  of  producing  the  extra  kilowatt-hours. 
In  a  water-power  plant  the  efficiency  is  an  unimportant  feature; 
in  a  steam  plant,  where  the  cost  of  fuel  is  high,  it  is  quite  im- 
portant. The  floor  space  taken  up  by  a  rotary  converter  and  its 
transformers  is  about  25  per  cent  greater  than  that  taken  up  by 
a  two-bearing  motor-generator  set.  The  floor  space  is  only  of 
importance  in  the  case  of  a  sub-station  in  a  city  where  real  estate 
is  valuable,  and  in  such  cases  the  transformers  could  be  placed 
in  a  gallery  over  the  rotaries  if  desired.  As  the  rotaries  them- 
selves only  take  up  about  two-thirds  the  floor-space  of  a  motor- 
generator  set,  the  advantage  would,  with  this  arrangement,  be 
with  them. 

Operating  Characteristics. — The  relative  desirability  of  rotary 
converters  and  motor-generators  from  the  operating  point  of 


ROTARIES  AND   MOTOR-GENERATORS  49 

view  depends  upon  the  question  of  their  reliability  and  flexibility 
of  operation.  As  regards  flexibility,  the  motor-generator  is  of 
course  by  far  the  better.  With  motor-generators  the  power- 
factor  of  the  motor  may  be  adjusted  to  unity,  or  if  desired  a 
leading  current  may  be  introduced  into  the  line  without  affecting 
the  operation  of  the  motor,  while  the  voltage  on  the  direct 
current  side  may  be  adjusted  within  wide  limits  either  by  use 
of  the  shunt  rheostat,  or  by  compounding.  Neither  of  these 
adjustments  can  be  applied  conveniently  to  a  rotary. 

In  a  rotary  converter  the  ratio  of  the  voltage  on  the  direct 
current  side  to  that  on  the  alternating  current  side  is  practically 
constant;  that  is,  any  drop  or  rise  of  voltage  on  the  line  affects 
proportionally  the  direct  current  voltage  of  the  rotary.  In 
attempting  to  regulate  the  power  factor  of  the  rotary  or  to  in- 
troduce leading  or  lagging  currents  into  the  line  by  varying  the 
field  strength,  we  are  liable  to  alter  the  alternating  current 
voltage  at  the  end  of  the  line,  and  hence  to  affect  the  direct 
current  voltage  of  the  rotary.  Variation  of  the  shunt  current 
within  wide  limits  is  also  objectionable,  because  it  often  has  a 
tendency  to  produce  hunting. 

Theoretically  a  rotary  converter  can  be  compounded  or 
over-compounded,  or  rather  the  line  can  be  over-compounded, 
producing  the  same  effect  on  the  direct  current  terminal  voltage 
as  if  the  rotary  itself  was  over-compounded.  This  may  be  done 
by  introducing  an  artificial  self-induction  into  the  line,  and 
producing  by  means  of  a  series  winding  on  the  converter  fields 
a  leading  current  approximately  proportional  to  the  load  on  the 
rotary.  This  leading  current  will  then  raise  the  voltage  of  the 
line  because  of  the  self-induction  present.  This  compounding  is, 
however,  at  best  a  rough  method  and  can  be  used  only  on  sys- 
tems in  which  exactness  of  voltage  is  unimportant,  as  it  is  some- 
what difficult  to  adjust  the  self-induction  and  the  series  winding 
to  give  the  required  effect.  With  this  method  the  power  factor 
of  the  rotary  and  that  of  the  system  are  also  varied  within  wide 
limits  as  the  load  varies,  and  a  change  of  voltage  affects  all  other 
machinery  on  the  line.  So  that  the  cases  are  limited  in  which 

4 


50  ROTARIES  AND   MOTOR-GENERATORS 

this  method  can  be  used  to  regulate  the  direct-current  voltage 
on  a  rotary. 

A  method  of  voltage  control  with  rotary  converters  is  used 
on  some  of  the  Edison  systems.  An  induction  regulator  is 
inserted  in  the  alternating  current  circuit  between  the  trans- 
former and  the  rotary,  and  is  usually  controlled  from  a  switch- 
board by  means  of  a  pilot  motor.  Such  a  regulator  increases 
the  cost  of  the  apparatus  about  20  per  cent,  decreases  the  total 
efficiency  about  1  per  cent,  adds  about  one-third  to  the  floor 
space  required  by  the  transformers,  and  introduces  additional 
complications  into  the  system.  Usually,  therefore,  the  necessity 
for  employing  an  induction  regulator  is  a  strong  argument 
against  the  use  of  rotary  converters  in  the  particular  installation 
considered. 

A  rotary  converter  is  more  liable  to  hunt  and  to  flash  over  on 
short  circuits,  and  is  a  somewhat  more  complicated  piece  of 
apparatus  than  a  motor-generator  set.  On  the  other  hand,  a  syn- 
chronous motor  wound  for  a  pressure  of  over  6,600  volts  is  not  so 
reliable  as  a  transformer  wound  for  the  same  voltage.  Generally 
speaking,  from  the  point  of  view  of  reliability  of  operation, 
there  is  little  choice  between  25  cycle  rotaries  and  25  cycle 
motor-generators,  although  in  a  60  cycle  installation  the  advan- 
tage is  decidedly  in  favor  of  the  motor-generator.  There  is  no 
doubt  that  satisfactory  60  cycle  rotaries  can  be  made  up  to 
600  volts,  but  their  design  is  more  difficult  than  that  of  25  cycle 
rotaries,  so  that  it  is  natural  they  should  require  more  attention 
than  motor-generator  sets. 

Motor-generators  have  another  advantage  over  rotary  con- 
verters in  that  they  are  not  so  liable  to  hunt.  Of  course,  hunt- 
ing can  be  prevented,  but  not  usually  without  introducing  some 
corresponding  disadvantage.  Dampers  may  be  placed  on  the 
pole  faces,  but  with  the  disadvantage  of  causing  some  loss  in  ef- 
ficiency; or  extreme  uniformity  of  engine  speed  may  be  obtained 
at  the  expense  of  a  heavy  fly-wheel;  while  the  small  line  drop 
that  is  usually  found  necessary  for  the  reliable  parallel  operation 
of  rotary  converters  requires  considerable  expense  for  conduc- 


ROT  ARIES  AND   MOTOR-GENERATORS  51 

tors.  Synchronous  motors  are  not  so  liable  to  hunt  as  rotary 
converters,  and  the  conditions  that  are  good  enough  to 
insure  the  satisfactory  parallel  operation  of  alternators  are 
usually  all  that  are  required  to  prevent  hunting  in  synchronous 
motors. 

From  the  operating  engineer's  standpoint,  a  motor-generator 
is  preferable  to  a  rotary  converter  in  almost  every  respect, 
except  as  to  efficiency  and  cost ;  and  even  as  to  cost  a  motor- 
generator  is  the  cheaper  for  low  voltages  and  large  outputs. 
Consequently,  when  comparatively  cheap  medium  size  units 
are  wanted,  and  close  voltage  regulation  is  unimportant,  rotary 
converters  are  used.  But  when  large  units  are  desired,  and 
the  voltage  regulation  is  important,  as  in  incandescent  lighting, 
motor-generators  are  employed.  This  applies  to  both  60  and  25 
cycles. 

So  far,  rotary  converters  have  only  been  considered  for 
transforming  from  alternating  current  to  direct  current.  In 
regard  to  inverted  rotary  converters;  that  is,  rotaries  for  trans- 
forming from  direct  current  to  alternating  current,  almost  the 
same  remarks  apply.  In  addition,  however,  inverted  rotaries 
are  subject  to  another  disadvantage:  that  the  power  factor  of 
the  load  on  the  alternating  current  side  affects  the  magnetic 
flux,  and  in  consequence  the  speed  and  frequency  of  the  ro- 
tary. A  heavy  inductive  load  on  the  alternating  current  side 
tends  to  make  the  rotary  run  away.  This,  of  course,  can  be 
prevented  by  an  automatic  speed  limit  device;  or,  to  a  cer- 
tain extent,  by  separately  exciting  the  rotary  from  an  under 
saturated  exciter  which  it  drives  mechanically,  or  by  making 
the  armature  of  the  rotary  very  weak  in  comparison  with  its  field 
magnet.  But  these  devices  are  makeshifts,  and  none  of  them 
can  keep  the  speed  absolutely  constant  under  these  conditions. 
And  when  the  speed  varies  it  also  causes  variation  in  the  speed 
of  all  induction  and  synchronous  motors,  driven  from  the  rotary, 
which  is  highly  objectionable.  This,  combined  with  its  other 
faults,  makes  an  inverted  rotary  usually  less  desirable  than  a 
motor-generator. 


52  ROTARIES  AND   MOTOR-GENERATORS 

Design  of  Rotary  Converters  and  Motor-Generators. — Generally 
speaking,  and  within  reasonable  limits,  the  higher  the  speed  of 
a  machine  the  less  is  its  cost,  so  that  it  is  to  the  interest  of  the 
manufacturer  to  run  at  as  high  a  speed  as  possible.  (See  NOTE 
p.  64.)  The  permissible  speed  for  any  alternator  is  limited 
only  by  mechanical  considerations,  while  the  maximum  speed 
at  which  a  direct- current  generator  or  a  rotary  converter 
of  a  given  output  can  be  run  is  limited  by  the  operating  charac- 
teristics in  regard  to  sparking.  Given  the  approximate  speed  at 
which  a  direct-current  machine  will  run,  the  number  of  poles 
which  it  should  have,  is  fixed  within  narrow  limits  by  questions 
of  sparking  and  economy  of  design.  As  the  number  of  poles 
and  speed  determine  the  frequency,  it  is  easily  seen  how  the 
choice  of  speed  for  which  a  rotary  of  given  output,  frequency, 
and  voltage  may  be  built  is  limited. 

Suppose  a  1,000  K.W.,  25  cycle,  600  volt  rotary  is  to  be 
designed:  to  insure  the  cheapest  machine  the  number  of  poles 
must  be  as  few  as  possible  so  that  the  speed  can  be  high.  There 
are  1,670  amperes  to  commutate;  this,  to  a  great  extent,  deter- 
mines the  number  of  poles.  On  laying  out  the  design,  it  is 
found  that  8  or  10  poles  will  suffice,  but  that  12  poles  will  be 
more  conservative.  Let  250  rev.  per  min.  be  decided  upon. 
Assuming  a  pole-pitch  of  21  in.,  we  obtain  an  armature  diameter 
of  80  in.,  24  slots  per  pole,  two  coils  per  slot,  length  of  armature 
core  13.5  in.  The  slots  are  comparatively  narrow,  only  0.4  in. 
wide,  so  that  solid  pole-faces  or  copper  dampers  can  be  used. 
This  makes  a  very  good  machine.  Fig.  18  shows  such  a 
machine. 

If  a  1,000  K.W.,  600  volt  motor-generator  is  to  be  designed, 
the  most  suitable  number  of  poles  on  the  direct  current  generator 
can  be  decided  on,  and  the  speed  may  be  made  as  high  as  is 
consistent  with  good  commutation.  The  speed  may  be  as  high 
as  300  rev.  per  min.,  but,  as  in  the  case  of  the  rotary,  a  more 
conservative  machine  would  result  if  the  speed  were  kept  dowrn 
to  250  rev.  per  min.  Twelve  poles  is  a  suitable  number,  and 
as  a  somewhat  better  sparking  constant  is  required  than  in  a 


ROTARIES  AND  MOTOR-GENERATORS 


53 


rotary,  the  armature  is  built  with  a  slightly  larger  diameter. 
An  armature  of  80  in.  diameter,  with  10  slots  per  pole  and  three 
coils  per  slot,  is  satisfactory.  The  width  of  slot  is  not  limited,  as 
laminated  pole-faces  are  to  be  used,  so  that  a  wide  slot  can  be 
adopted  with  its  consequent  reduction  in  the  self-induction  of 
the  commutated  coil.  This  gives  an  armature  length  of  10.5 
in.,  airl  also  makes  a  very  good  machine. 


FIG.  18.— 1,000  K.W.  600  Volt,  25  Cycle  Rotary  Converter. 

This  is  one  instance  in  which  we  find  that  the  rotary  con- 
verter and  motor-generator  will  operate  at  the  same  speed. 
In  this  case,  the  cost  of  the  rotary  and  transformers  will  be  less 
than  the  corresponding  motor-generator  set. 

Suppose,  on  the  other  hand,  a  1,500  K.W.,  275  volt,  25  cycle 
machine  is  to  be  designed.  It  is  found  that  the  minimum 
number  of  poles  for  a  rotary  of  this  output  and  voltage  is  about 


54  ROTARIES   AND   MOTOR- GENERATORS 

20.  This  gives  a  20  pole  rotary  at  150  rev.  per  min.,  with  an 
armature  130  in.  diameter,  13  in.  long,  24  slots  per  pole,  and  one 
coil  per  slot. 

The  corresponding  generator  for  the  motor-generator  set 
may  be  run  at  250  rev.  per  min.  by  making  it  with  18  poles. 
This  gives:  armature  110  in.  diameter,  9.5  in.  long,  14  slots 
per  pole,  and  two  coils  per  slot.  In  this  case  the  cost  of  the 
motor-generator  will  be  less  than  that  of  the  corresponding 
rotary  and  transformers,  on  account  of  higher  speed  at  which 
the  motor-generator  can  be  run.  Such  a  motor-generator  is 
shown  in  Fig.  19. 

Speaking  generally  from  the  designer's  standpoint,  there  is 
little  to  choose  between  the  difficulty  of  designing  a  rotary 
converter  and  that  of  designing  a  direct  current  generator  of 
the  same  output,  speed,  and  voltage.  The  design  of  a  rotary 
is  subject  to  more  limitations  than  that  of  a  direct  current 
generator.  The  number  of  commutator  segments  per  pair  of 
poles  must  be  divisible  by  the  number  of  phases,  and  the  number 
of  slots  per  pair  of  poles  should  preferably  be  also  divisible  by 
the  same  number.  Also  the  relative  dimensions  of  the  slot  and 
air  gap  are  limited  by  the  fact  that  eddy  currents  must  be 
avoided  in  the  solid  pole-faces,  or  copper  dampers,  which  are 
usually  employed  to  prevent  hunting.  On  the  other  hand,  the 
absence  of  armature  reaction  in  a  rotary  converter  is  a  consider- 
able point  in  its  favor  as  regards  tendency  to  sparking. 

Investigating  the  conditions  of  commutation  in  a  direct- 
current  generator  by  means  of  a  pilot  brush,  when  the  machine 
is  operating  at  full  rated  output  with  brushes  set  in  the  normal 
position,  it  is  generally  found  that  resistance  commutation  is 
taking  place;  that  is,  the  brushes  are  advanced  just  far  enough 
for  the  armature  cross  magnetization  field  to  neutralize  the 
direct  field  due  to  the  magnets  at  the  point  of  commutation. 
As  the  load  on  the  machine  is  increased,  the  increased  armature- 
reaction  causes  the  resultant  field  at  the  point  of  commutation 
to  become  of  the  opposite  sign  to  that  which  would  be  required 
for  perfect  commutation,  thus  tending  to  make  the  brushes 


ROTARIES   AM)    MOTOR-GENERATORS  5o 

spark.  At  the  same  time  the  increased  current,  which  has  to 
be  commutated,  also  has  a  tendency  to  make  the  brushes  spark 
unless  the  resistance  of  the  brush  contact  is  sufficiently  high. 
Thus  assuming  the  generator  is  delivering  the  maximum  current 
that  the  brushes  could  commutate  by  the  varying  resistance  of 
their  contacts  if  they  were  in  a  zero  resultant  magnetic  field, 
any  increase  of  load  on  the  direct  current  generator  will  cause 
the  brushes  to  spark,  for  two  reasons:  first,  because  of  increased 


899  B 


FIG.  19.— 1,500  K.W.  6,600  Volt,  3  Phase,  25  Cycle  and  300  Volt  D.C. 
250  R.P.M.  Motor-Generator. 

armature-reaction;  second,  because  the  current  becomes  too 
great  to  be  taken  care  of  by  resistance  commutation,  even  assum- 
ing no  armature-reaction. 

In  a  rotary  converter,  on  the  other  hand,  the  armature- 
reaction  effect  is  not  present,  and  the  brushes  may  bo  assumed 
at  all  times  to  be  either  in  a  neutral  field  or  in  one  that  is  help- 
ing the  commutation.  The  result  of  this  is  that  a  direct  current 
machine  operated  as  a  rotary  converter  will  carry,  as  regards 
sparking,  heavier  overloads  with  fixed  brushes  than  will  the 


56  ROTARIES  AND   MOTOR-GENERATORS 

same  machine  as  a  generator.  The  sparking  does  not  appear 
to  increase  so  rapidly  in  a  rotary  converter  when  the  load  is 
raised.  The  result  of  this  is  that  the  sparking  constant  in  a 
rotary  is  usually  permitted  to  be  about  25  per  cent  higher  than 
in  a  generator,  and  in  consequence  of  this  a  rotary  converter 
may  be  designed  for  a  lower  peripheral  speed,  and  with  a 
longer  armature  core  than  the  corresponding  generator. 

There  being  no  resultant  armature-reaction  in  a  rotary 
converter,  some  manufacturers  design  such  machines  with  a 
high  armature-reaction  and  low  volts  per  commutator  bar; 
that  is,  with  a  strong  armature  and  weak  field.  This  design 
requires  less  material  in  the  construction  and  tends  to  lowrer  the 
cost  of  the  machine.  But  this  saving  is  not  so  pronounced  as 
one  might  think  at  first  sight,  as  the  labor  cost  is  increased  quite 
materially  when  the  number  of  coils  on  the  armature  and  the 
number  of  commutator  bars  are  increased.  In  addition,  a 
strong  armature  is  not  conducive  to  good  operation  in  a  rotary 
converter;  it  tends  to  make  the  brushes  flash  badly  or  even  to 
flash  over  when  starting  from  the  alternating  current  side,  or 
when  hunting,  and  it  also  reduces  the  synchronizing  power  of 
the  machine.  Considering  the  question  from  all  points  of  view, 
it  is  usually  found  most  satisfactory  to  design  rotary  converters 
with  about  the  same  armature  reaction  and  volts  per  bar  as  the 
corresponding  direct  current  machines. 

Rotary  Converter  Armature  Winding. — The  copper  loss  in 
the  armature  of  a  polyphase  rotary  converter  is  usually  consider- 
ably less  than  in  the  corresponding  direct  current  generator, 
so  that  such  rotaries  are  often  designed  with  a  much  smaller 
cross-section  of  copper  than  would  be  used  in  a  generator. 
This  is  bad  practice.  The  copper  loss  in  a  rotary  converter 
armature  is  not  equally  distributed,  the  loss  in  the  bars  nearest 
the  collector  leads  being  usually  much  greater  than  in  those 
midway  between  the  leads.  And  although  the  difference  in 
temperature  at  the  end  of  a  temperature  run  cannot  generally 
be  detected  by  a  thermometer,  the  difference  is  very  appreciable 
in  the  case  of  sudden  overload  at  a  low  power  factor.  And  cases- 


ROTARIES  AND   MOTOR-GENERATORS  57 

are  on  record  where  the  armature  bars  connected  to  the  leads 
in  a  large  rotary  have  been  fused  before  the  other  bars  got 
dangerously  hot.  For  heavy  railway  work  the  section  of  the 
copper  in  a  rotary  armature  ought  to  be  at  least  equal  to  that 
in  the  corresponding  generator.  As  regards  heating,  six  phase 
rotary  converters  have  an  advantage  over  two  or  three  phase, 
but  as  they  result  in  extra  complications  in  the  cables  and 
switchboard  they  are  seldom  employed  except  in  large 
units.  In  any  case  a  six  phase  rotary  may  have  to  operate 
three  phase  at  some  time,  so  it  should  be  designed  simply  as  a 
three  phase  machine  with  three  extra  collector  rings.  This 
being  the  case,  the  remarks  before  made  in  regard  to  the  section 
of  the  armature  copper  and  heating  of  conductors  also  apply 
to  six  phase  machines. 

One  advantage  sometimes  claimed  for  the  six  phase  rotary 
is  that  having  a  greater  number  of  equipotential  connections 
on  the  armature  than  either  a  two  or  three  phase,  there  is  a 
greater  tendency  towards  equality  in  distribution  of  current 
between  the  various  sets  of  brushes  on  the  commutator.  This 
would  be  true  if  the  collector  rings  were  the  only  equipotential 
connections  on  the  armature.  But  in  addition  to  the  collector 
rings,  the  armature  winding  of  the  modern  high  speed  rotary  is 
usually  provided  with  an  equipotential  connection  for  every 
second  slot,  while  in  60  cycle  or  large  25  cycle  rotaries  where  the 
commutating  conditions  are  more  severe,  one  connection  to  every 
slot  is  frequently  employed.  So  it  is  obvious  that,  in  this  re- 
spect at  least,  the  six  phase  rotary  as  usually  constructed  pre- 
sents no  advantage  over  the  two  or  three  phase.  Undoubtedly 
the  larger  the  number  of  phases  in  the  armature  winding  of  a 
rotary  converter,  the  more  accurately  and  uniformly  do  the 
alternating  and  direct  currents  in  the  conductors  neutralize 
one  another,  and  in  consequence,  the  less  the  pulsation  of  the 
armature  reaction  and  the  less  the  variation  in  the  commutating 
conditions.  But  with  the  modern  well  proportioned  rotary 
converter,  this  variation  and  pulsation  is  not  a  serious  matter, 
so  that  it  should  not  influence  the  choice  of  the  number  of  phases 


58  ROTARIES  AND  MOTOR-GENERATORS 

any  more  than  the  necessity  for  additional  equipotential  con- 
nections should. 

An  important  feature  in  the  design  of  the  armature  of  a 
rotary  converter  is  often  overlooked,  and  that  is  equality  or 
balancing  of  the  phases.  If  the  windings  of  the  different  phases 
on  a  rotary  armature  are  not  all  exactly  equal,  and  placed 
on  the  armature  in  an  exactly  similar  and  symmetrical  position 
with  regard  to  one  another,  then  the  phases  will  be  unbalanced, 
with  the  result  that  the  load  will  not  be  balanced  among  the 
phases,  and  that  there  will  be  a  greater  tendency  to  hunting. 
If  the  three  ammeters  in  the  three  phases  of  such  a  rotary  are 
watched,  the  load  can  be  seen  changing  from  one  phase  to  an- 
other. This  is  the  reason  why  rotary  converters  with  series 
wound  armatures  are  usually  unsatisfactory.  It  is  often  im- 
possible to  balance  the  phases.  A  6  pole,  three  phase  rotary 
having  a  series  wound  armature  with  224  coils,  must  have  the 
rings  connected  to  coils  1-26-51.  An  8  pole  three  phase  rotary 
having  a  multiple  wound  armature  with  520  coils,  must  have 
its  rings  connected  to  coils  1-44-86.  The  phases  of  both 
these  armatures  are  unbalanced.  To  have  the  phases  perfectly 
balanced  the  number  of  commutator  bars  per  pair  of  poles 
should  not  only  be  divisible  by  the  number  of  phases,  but  the 
number  of  slots  per  pah-  of  poles  should  also  be  divisible  by  the 
same  number.  The  reason  for  this  is  that  to  have  the  different 
phase  windings  all  symmetrically  and  similarly  placed  on  the 
armature,  all  the  coils  that  are  connected  to  the  phase  leads 
must  be  in  the  same  relative  positions  in  their  slots. 

Hunting  of  Rotary  Converters  and  Synchronous  Motors. — 
Rotary  converters  are  more  liable  to  hunt  than  synchronous 
motors.  Generally  speaking,  conditions  of  operation  and  design 
that  will  enable  two  alternators  to  operate  satisfactorily  in 
parallel  without  hunting,  will  also  enable  one  of  them  to  operate 
satisfactorily  as  a  synchronous  motor  when  driven  by  the  other 
machine  under  the  same  conditions.  A  single  rotary  driven 
from  an  alternating  current  system,  will  not  operate  under 
given  conditions  quite  so  well  as  regards  hunting,  as  a  motor- 


ROTARIES  AND   MOTOR-GENERATORS  59 

generator.  The  armature  reaction  of  a  rotary  converter  is 
considerably  higher  in  proportion  than  that  of  a  synchronous 
motor.  Therefore,  its  synchronizing  power  is  less  and  the  fact 
that  the  direct  current  side  is  so  intimately  connected  to  the 
alternating  current  side,  makes  it  peculiarity  sensitive  to  hunting. 
But  the  main  difficulties  with  rotaries  in  regard  to  hunting  are 
experienced  when  two  or  more  rotaries  are  running  in  parallel 
on  the  same  alternating  current  system,  and  feeding  into  the 
same  direct  current  system.  These  difficulties  are  especially 
marked  when  the  rotaries  are  running  in  parallel  upon  the  same 
alternating  current  and  the  same  direct  current  bus-bars. 
Under  similar  conditions  motor-generators  are  no  more  sen- 
sitive to  hunting  than  under  the  conditions  of  singly  operated 
units.  But  the  difficulties  with  rotaries  are  often  so  serious, 
that  operating  engineers  have  found  it  necessary  to  insist  that 
manufacturers  provide  damping  or  anti-hunting  devices  for  all 
machines  intended  to  be  connected  in  this  way,  and  also  to 
install  artificial  choke  coils  between  the  collector  rings  and  the 
alternating  current  bus-bars,  in  order  to  limit  the  interchange 
of  current. 

Artificial  damping  devices  of  various  types  and  forms  have 
been  tried,  but  the  only  one  in  extensive  use  at  the  present  time 
is  a  heavy  grid  of  copper  embedded  in  the  pole  face.  Another 
construction  now  in  use  for  accomplishing  the  same  result, 
consists  in  solid  pole  faces  so  shaped,  and  with  the  armature 
slots  and  air  gap  so  proportioned,  that  eddy  currents  due  to 
the  teeth  are  avoided.  As  far  as  can  be  seen  from  practical 
operation,  these  two  methods  of  preventing  hunting  seem 
equally  effective.  They  both  enable  rotaries  to  be  run  in 
parallel  satisfactorily,  as  long  as  the  variation  in  speed  of  the 
engines  and  the  pressure  drop  in  the  feeders  is  not  excessive. 
As  regards  the  relative  effects  of  a  copper  grid  damper  and  a 
solid  pole  face  upon  the  efficiency  under  working  conditions,  it 
is  difficult  to  speak  with  any  degree  of  accuracy  as  so  much 
depends  on  the  uniformity  of  speed  of  the  engines.  But  it  is 
probable  there  is  a  constant  loss  in  the  copper-grid  damper, 


60  ROTARIES  AND   MOTOR-GENERATORS 

when,  as  is  often  the  case,  it  is  used  with  large  armature  teeth 
and  small  air  gaps.  Solid  pole  faces  require  a  larger  air  gap  or 
narrower  slot,  which  in  turn  demands  more  copper  on  the 
magnets,  this  being  especially  the  case  in  60  cycle  converters. 
On  the  other  hand,  from  a  mechanical  point  of  view  it  is  a  nuis- 
ance to  have  to  attach  auxiliary  copper  grids  or  dampers  to 
any  pole  face,  while  the  cost  of  the  dampers  themselves  is  not 
insignificant.  Generally  speaking,  then,  there  is  little  choice 
between  solid  pole-faces  and  auxiliary  dampers,  so  it  would 
perhaps  be  best  to  advise  the  use  of  solid  pole-faces  in  all  cases, 
as  they  are  simpler  and  more  mechanical. 

Mechanical  Design. — Though  rotary  converters  have  been 
used  for  quite  a  number  of  years,  yet  their  detail  design  seems 
to  have  received  less  care  than  has  been  given  to  generator 
design.  A  rotary  in  a  sub-station  carrying  a  street  railway 
or  interurban  load  is  usually  subjected  to  rough  treatment, 
and  consequently  should  be  of  robust  design.  All  parts  should 
be  as  accessible  as  possible  in  order  to  facilitate  repairing. 

Two  features  in  the  design  of  rotaries  that  are  often  faulty 
are  the  alternating  current  collector  gear  and  leads,  and  the 
starting  resistance  used  for  starting  from  the  direct  current  side. 
The  alternating  current  end  of  a  rotary  is  often  designed  sim- 
ilarly to  the  old  revolving-armature  alternators;  that  is,  the 
leads  are  strap-copper  soldered  to  the  armature  conductor  and 
attached  to  the  armature  end-plate  with  a  few  cleats.  While 
the  collector-rings  are  mounted  solid  upon  the  shaft,  separated 
and  insulated  by  fiber  or  wood  discs  and  bushings,  the  leads 
being  embedded  in  this  insulation  where  they  pass  through  or 
are  connected  to  the  rings.  This  construction  is  shown  in  Fig.  20, 
and  it  is  hardly  necessary  to  state  that  it  is  unsatisfactory  for 
heavy  railway  work.  Solid  copper  leads,  unless  very  carefully 
and  solidly  cleated  to  the  armature  end  plates,  are  liable 
to  break  due  to  vibration,  while  soldered  joints  are  liable  to  melt 
under  overloads.  Collector-rings  mounted  solid  on  the  shaft 
are  liable  to  break  down  due  to  warping  or  cracking  of  the 
insulation,  and  to  get  hot  on  overloads  due  to  poor  cooling 


NOTARIES  AND   MOTOR -GENERATORS 


61 


facilities.  Also  when  the  rings  are  insulated  by  wooden  discs 
projecting  between  the  rings  they  cannot  be  easily  turned  off 
when  they  become  cut  or  grooved  by  the  metal  brushes.  The 
most  substantial  construction  is  probably  to  use  cable  for  the 
leads  and  to  connect  them  to  the  winding  by  special  lugs  riveted 
and  silver-soldered  to  the  armature  conductors.  The  rings  should 
be  carried  on  arms  projecting  from  a  spider  and  should  be  freely 


FIG.  20. — Rotary  Converter  Armature  and  Collector-Rings. 

open  to  the  air  for  cooling,  and  be  easily  accessible  for  turning 
off  whenever  they  become  grooved  or  uneven  from  wear.  In  fact 
they  should  be  designed  exactly  like  the  collector  gear  of  an  up- 
to-date  revolving-field  alternator.  When  this  construction  is 
adopted  a  temperature  rise  of  over  15  degrees  is  rarely  attained 
on  normal  load.  Such  collector-rings  are  shown  in  Fig.  21. 


ROTARIES  AND   MOTOR-GENERATORS 


Regarding  a  starting  resistance  for  starting  a  rotary  or  a 
motor-generator  from  the  direct  current  side,  it  is  often  for- 
gotten that  such  work  is  much  more  severe  than  starting  a 
motor.  A  rotary  or  a  motor-generator  has  to  be  run  up  to  speed 
and  then  synchronized,  and  when  synchronizing  the  speed  has 


r 


, 


FIG.  21. — A.C.  Collector-Rings  on  Rotary  Converter. 


to  be  exact.  If  the  voltage  on  the  direct  current  system  is  vary- 
ing, it  often  requires  several  minutes  to  synchronize,  and  if  it 
is  varying  suddenly,  the  speed  can  only  be  adjusted  by  means 
of  the  starting  resistance  because  shunt  control  is  not  quick 
enough.  Starting  resistances  for  rotaries  or  motor-generators 
should  be  designed  so  that  the  last  steps  can  be  kept  in  circuit 


ROTARIES  AND   MOTOR-GENERATORS  63 

for  at  least  five  minutes  without  overheating.  An  oil-cooled 
starting  resistance  that  has  been  designed  by  the  National 
Electric  Co.  for  this  work  is  shown  in  Fig.  22.  The  resistance 
coils  are  of  iron  wire  supported  on  porcelain  insulators  and 
brazed  to  heavy  brass  terminals.  It  is  designed  so  that  the 
temperature  rise  of  the  oil  will  be  150°  Cent,  in  five  minutes 
with  all  the  resistance  coils  carrying  their  maximum  rated  cur- 
rent. This  type  of  resistance  gives  excellent  results,  and  has  the 
additional  advantages  of  being  quite  cheap  and  almost  fireproof. 
The  usual  starting  switch,  with  overload  and  no-load  release, 


FIG.  22. — Starting   Resistance    for    Rotary   Converter  or   Motor-Generator. 

is  neither  necessary  nor  desirable  for  rotary  converter  work; 
it  is  too  complicated  and  expensive  and  not  reliable  enough 
for  such  duty.  A  standard  multiple-contact  switch  is  all  that 
is  required.  Such  a  switch  requires  very  little  space  and  may 
be  mounted  on  the  rotary  panel. 

This  paper  has  not  been  written  with  the  idea  of  advocating 
the  use  of  rotary  converters  instead  of  motor-generators,  or 
vice  versa,  but  more  with  the  idea  of  comparing  them  generally, 
their  advantages  and  disadvantages,  and  of  pointing  out  some 
of  the  characteristic  features  of  each  machine.  The  question 
as  to  which  type  of  machine  should  be  used  in  any  given  case 


64  ROTARIES  AND   MOTOR- GENERATORS 

can  only  be  decided  after  every  feature  of  the  situation  has  been 
duly  considered.  Broadly  speaking,  however,  the  tendency 
to-day  is  toward  motor-generator  sets  in  lighting  systems  and 
rotary  converters  on  traction  systems.  This  seems  to  be  per- 
haps the  most  rational  conclusion. 

Motor-generators  and  rotary  converters,  all  things  con- 
sidered, are  more  difficult  to  design  and  build  than  the  ordinary 
standard  engine-type  generator.  They  are  high-speed  machines 
and  usually  operate  in  sub-stations  where  the  conditions  of 
operation  and  the  supervision  are  not  of  the  best.  The  question 
of  their  reliability  in  continuous  operation  should  therefore 
receive  the  most  careful  consideration  from  the  designer  and 
manufacturer;  and  as  suggestions  and  criticisms  from  operating 
engineers  are  of  the  utmost  value  they  should  always  be 
welcomed  and  investigated,  in  order  to  determine  whether  or 
not  they  contain  features  valuable  enough  to  warrant  the 
modification  of  standard  designs. 

NOTE  [Dec.,  1910] 

In  the  past  two  years  the  standard  speed  for  rotary  con- 
verters and  motor-generators  has  appreciably  increased,  this 
being  due  partly  to  improvements  in  mechanical  design;  partly 
to  the  development  of  commutating  poles  for  direct  current 
generators  and  to  a  certain  extent  for  rotary  converters;  and 
partly  to  our  increased  knowledge  of  power  system  phenomena 
and  to  the  education  of  po\ver  station  engineers,  which  has  per- 
mitted closer  commercial  designing.  The  present  commutating 
pole  units  require  careful  adjustment,  but,  after  adjustment 
are  much  less  sensitive  to  operating  conditions,  and  require 
less  attention  and  maintenance. 

As  stated  above,  increasing  the  speed  of  any  unit  tends  to 
decrease  the  cost,  though  this  is  true  only  within  certain  limits. 
This  is  exemplified  by  the  direct  current  generators  which  are 
now  built  for  operation  at  the  very  high  speed  required  for 
direct  connection  to  steam  turbines,  and  whose  cost  is,  on 
account  of  the  mechanical  difficulties  of  construction  due  to  the 


ROTARIES  AND  MOTOR-GENERATORS 


65 


high  speed,  appreciably  greater  than  that  of  the  corresponding 
medium  speed  unit.  Given  the  approximate  speed  at  which  a 
direct  current  generator  is  to  operate,  and  adopting  a  commutat- 
ing  pole  construction,  the  number  of  poles  which  the  machine 
should  have  is  fixed  within  narrow  limits  by  questions  of  cost, 
efficiency,  and  mechanical  design.  If  we  carefully  design  a 
unit  of  given  rating,  whether  A.C.,  or  D.f.  (with  commutating 
poles),  for  a  number  of  different  speeds,  and  figure  the  efficiency 

STANDARD   ROTARY   CONVERTER   AND   MOTOR- 
GENERATOR   SPEEDS   (1010) 


25   C'YCLKS 


MOTOR-GENERATORS. 


NOTARY    CO.VVKRTKR*. 


K.W. 

250  Volts. 

600  Volts. 

i.'.-,o  Volts. 

GDO  Volts. 

250 

750 

750 

750 

750 

500                       750 

750 

500 

500 

1000                       500 

500 

300 

375 

1500 

375 

375 

214 

250 

2000 

3<X) 

300 

167 

187 

60   CYCF.liS 


250 

1200 

1200 

1200         1200 

500         720 

720 

720 

900 

1000         514         .'.I  1 

514 

600 

1500         300         360 

360 

360 

2000         300         300 

240 

240 

and  cost  of  manufacture  for  each  design,  we  find  that  with  in- 
creased speed  the  efficiency  improves  and  the  cost  decreases  till 
a  certain  speed,  dependent  on  the  rating,  is  reached;  and  that 
if  the  speed  is  still  further  increased,  the  efficiency  is  reduced 
and  the  cost  increased.  There  is  a  similar  relationship  between 
the  number  of  poles,  and  the  efficiency  and  cost.  The  smaller 
the  number  of  poles,  the  lower  the  frequency  and,  generally 
speaking,  the  larger  the  flux  per  pole,  and  in  consequence  the 
heavier  the  magnetic  circuit.  On  the  other  hand,  when  the 
number  of  poles  is  excessive,  the  frequency  is  high  and  the 
5 


66 


ROTARIES  AND  MOTOR  GENERATORS 


diameter  of  the  armature  becomes  very  large;  while  the  cost 
of  the  armature  and  field  winding,  and  in  a  direct  current  ma- 
chine, the  cost  of  commutator  and  brush  gear,  also  increases 
rapidly.  The  most  economical  and  efficient  number  of  poles 


FIG.  23.— 3,000  K.W.,  600  Volt,  25  Cycle  Rotary  Converter. 

to  adopt  for  any  machine  of  given  speed  and  rating,  whether 
A.C.  or  D.C.,  can  be  readily  determined  by  a  few  trial  designs, 
and  any  change  in  this  number  usually  results  in  increased 
cost  and  decreased  efficiency. 

Adopting  a  modern  commutating  pole  design  for  the  D.C. 


ROTARIES   AND   MOTOR-GENERATORS  67 

unit  of  a  motor-generator  set  of  given  rating,  we  can  select  the 
most  suitable  number  of  poles  and  speed  without  any  restrictions 
other  than  those  mentioned  above.  But  with  a  rotary  converter 
the  frequency  is  fixed,  so  that  the  number  of  poles  must  vary 
inversely  with  the  speed.  This  restriction  is  more  serious  the 
lower  the  frequency,  as  we  are  dealing  with  a  smaller  number  of 
poles,  so  that  it  is  often  easier  to  develop  an  economical  design 
for  a  motor-generator  than  it  is  for  a  rotary  converter.  The 
recent  developments  in  commutating  pole  designs  for  direct 
current  machines  have  allowed  an  appreciable  increase  in  speed 
of  motor-generator  sets  compared  with  rotary  converters,  so 
that  at  the  present  time  the  comparison  in  cost,  efficiency,  and 
floor  space,  between  these  two  types  of  units  is,  if  anything, 
more  favorable  to  the  motor-generator  than  it  was  in  1905. 


[Presented  before  the  American  Institute  of 
Electrical  Engineers,  May  28,  1906.] 


SHUNT  AND  COMPOUND  WOUND  ROTARY  CONVERT- 
ERS FOR  RAILWAY  WORK 


THE  question  of  shunt  or  compound  excitation  for  rotary 
converters  has  been  discussed  so  much  that  it  is  of  interest 
to  compare  the  relative  merits  of  the  two.  As  the  shunt 
winding  is  obviously  the  simplest,  cheapest,  and  most  convenient 
way  of  exciting  a  rotary  converter,  it  would  be  well  to  begin 
by  considering  why  a  compound  winding  is  ever  used. 

In  a  direct  current  circuit  it  is  often  an  advantage  to  have 
a  system  which  is  to  a  great  extent  self-regulating  as  regards 
voltage.  This  is  especially  the  case  where  the  load  changes 
frequently,  and  in  such  cases  a  compound- wound  generator  is 
used.  A  series  winding  on. the  generator  field-coils  tends  to 
make  the  voltage  at  the  generator  terminals  rise  as  the  load 
comes  on  the  machine,  and  this  rising  characteristic  is  employed 
to  counteract  the  increasing  voltage  drop  in  the  feeders  and  mains 
due  to  increasing  load.  By  changing  the  series  winding  on  the 
generator  the  rising  tendency  of  the  terminal  voltage  can  be 
varied  to  almost  any  extent. 

Compounding  of  Rotaries. — In  a  rotary  converter  the  ratio 
of  the  voltages  at  the  terminals  on  the  two  sides  of  the  machine 
is  approximately  constant,  and  independent  of  the  load  or  the 
magnitude  of  the  excitation;  that  is,  when  transforming  from 
alternating  current  to  direct  current,  the  direct  current  ter- 
minal voltage  bears  an  almost  constant  ratio  to  the  alternating 
current  terminal  voltage  under  all  conditions.  So  the  only 

69 


70          SHUNT  AND   COMPOUND  WOUND  ROTARIES 

way  of  varying  the  direct  current  terminal  voltage  is  to  vary 
the  alternating  current  voltage  supplied  to  the  machine.  If 
the  direct  current  voltage  is  to  rise  as  the  load  comes  on  the 
rotary,  then  the  alternating  current  voltage  which  is  supplied 
must  be  made  to  vary  in  the  same  way.  Assuming  that  approxi- 
mately constant  voltage  is  supplied  at  the  generator  end  of 
the  alternating  current  feeders,  and  that  the  circuit  between 
the  generator  and  the  rotary  converter  contains  sufficient  self- 
induction,  then  the  voltage  at  the  rotary  converter  end  of  the 
alternating  current  line  can  be  raised  or  lowered  by  intro- 
ducing a  leading  or  a  lagging  current  into  the  system.  A 
leading  or  lagging  current  can  be  introduced  into  the  system 
by  over  or  under  exciting  the  rotary.  So  by  putting  a  series 
winding  on  the  magnets  of  the  rotary  the  excitation  will  be  in- 
creased as  the  load  comes  on,  and  a  leading  current  approxi- 
mately proportional  to  the  load  introduced  in  the  alternat- 
ing current  system.  This  will  tend  to  raise  the  alternating 
current  voltage  supplied  to  the  rotary  and,  in  consequence,  the 
terminal  voltage  at  the  direct  current  side.  By  means  of  this 
system,  a  rotary  converter  can  be  compounded  in  a  manner 
similar  to  that  employed  for  compounding  a  direct  current 
generator,  or  rather  the  line  can  be  compounded  so  as  to  cause 
the  voltage  at  the  direct  current  terminals  of  all  the  rotaries  on 
the  line  to  increase  as  the  load  comes  on. 

Here  then  is  a  system  that  gives  automatic  control  of  the 
voltage  as  the  load  varies.  Such  a  system  is  obviously  ex- 
tremely useful  and  convenient.  Unfortunately  it  presents  a 
number  of  disadvantages  in  practice,  e.£. — a  series  winding 
is  required  on  the  rotary  magnets;  it  ifs  practically  always 
necessary  to  insert  artificial  self-induction  in  the  alternating 
current  line,  so  as  to  bring  its  reactance  up  to  the  required  value; 
and  extra  switchboard  arrangements  are  required.  This  results 
in  increased  complications  and  cost,  and  a  loss  of  efficiency. 
A  compound-wound  rotary  costs  about  5  per  cent  more  than  a 
shunt-wound,  and  reactance  coils  usually  cost  about  5  per  cent 
of  the  cost  of  the  rotary,  while  the  efficiency  of  the  system 


SHUNT  AND  COMPOUND  WOUND  ROTARIES          71 

is  lowered  probably  1  to  2  per  cent.  In  addition  the  system 
is  more  complicated,  and  in  consequence  more  liable  to  break 
down,  while  there  is  a  possibility  of  troubles  in  operation,  due 
to  the  fact  that  a  series  field  winding  on  a  rotary  converter 
is  a  source  of  danger  on  account  of  its  liability  to  reversal. 
When  starting  a  rotary  the  series  fields  can  be  short-circuited, 
and  the  danger  at  that  time  avoided ;  but  if  the  attendant  forgets 
the  short-circuiting  switch,  there  will  be  trouble.  And  if  at  any 
time,  when  the  series  coils  are  in  use,  the  alternating  current 
supply  fails  or  is  cut  off,  so  that  the  machine  is  left  connected 
to  the  direct  current  circuit,  the  series  coils  will  reverse  and 
it  is  likely  to  run  away;  especially  if  the  direct  current  voltage  is 
varying.  A  speed-limit  device  should  take  care  of  this,  but  as 
automatic  devices  usually  go  wrong  when  they  are  most  needed, 
the  fact  must  be  accepted  that  a  series  winding  introduces  a 
possible  source  of  serious  trouble  in  any  rotary  converter  system. 
Small  Sub-Stations. — The  automatic  compounding  obtained 
by  a  series  winding  and  reactance  coils  is  not  so  satisfactory  as 
would  be  expected,  because  the  compounding  of  all  the  rotaries 
is  proportional  to  the  total  load  on  the  line,  rather  than  to  the 
load  on  the  individual  rotaries.  We  would  expect  that  the  most 
useful  application  of  this  system  would  be  to  over  compound 
rotaries  in  a  small  sub-station,  so  as  to  compensate  for  a  large 
feeder  drop,  where  the  load  fluctuates  violently.  The  results 
in  such  a  case  are,  however,  not  always  satisfactory.  To  over 
compound  a  rotary,  and  to  change  the  voltage  at  the  direct 
current  side,  requires  that  the  magnetic  flux  in  the  field  magnets 
of  the  rotary  must  change;  as  solid  steel  field  magnets  or  copper 
dampers  are  always  used  in  rotaries  the  magnetism  cannot 
change  quickly,  and  there  is  therefore  often  a  considerable  tune 
lag  before  the  voltage  changes  to  correspond  with  the  change  in 
load.  The  result  of  this  is  that  when  the  load  is  varying  quickly 
the  voltmeter  needle  is  kept  wandering  aimlessly  about  the 
scale,  indicating  anywhere  from,  say,  500  to  650  volts,  the 
voltage  apparently  not  having  any  relation  to  the  load  on 
the  machine.  However,  when  the  rotary  is  flat-compounded 


72          SHUNT  AND  COMPOUND  WOUND    ROTARIES 

instead  of  over  or  under-compounded  the  result  is  more  satis- 
factory, as  the  natural  tendency  of  the  solid  poles  or  dampers 
is  to  hold  the  magnetism  and  voltage  constant.  Unfortunately, 
however,  small  sub-stations  are  usually  supplied  from  com- 
paratively small  power  stations;  and  with  a  varying  load  on 
the  rotaries  one  of  the  main  effects  noticed  in  the  sub-station 
is  the  fluctuation  of  the  speed  of  the  engines,  and  consequently 
in  the  speed  of  the  rotaries  and  the  alternating  current  voltage. 
The  effect  of  this  variation  in  speed  often  completely  masks 
all  results  from  the  compound  winding. 

In  addition,  this  method  of  compounding  is  often  a  nuisance. 
There  must  be  careful  adjustment  of  the  series  winding  and 
of  the  reactance-coils  at  the  sub-stations  before  obtaining  the 
desired  effect.  Then  if  the  rotaries  are  changed  to  another 
station,  or  if  the  line  conditions  are  changed,  the  adjustment 
has  to  be  made  again.  If  other  compound-wound  rotaries 
are  installed  in  the  same  sub-station,  there  is  trouble  to  adjust 
them  so  that  they  divide  the  load  properly  at  all  loads.  Equal- 
izer connections  are  used,  but  the  results  are  rarely  altogether 
satisfactory.  The  different  characteristics  of  the  rotaries,  and 
the  variation  in  the  brush-contact  resistance  or  temperature  of 
the  machines  all  tend  to  upset  the  adjustment.  If  shunt- wound 
rotaries  are  not  dividing  the  load  properly,  varying  the  field 
rheostat  will  quickly  adjust  the  load.  And  in  any  case  shunt- 
wound  rotaries,  like  shunt- wound  generators,  have  a  much 
greater  natural  tendency  to  divide  the  load  properly  than  have 
compound- wound  machines.  With  this  automatic  coiinpound- 
ing,  the  power-factor  of  the  rotary  system  varies,  arid  often 
varies  widely,  with  the  load.  The  power-factor  cannot  be  in- 
terfered with,  without  disturbing  the  complete  system  of  regula- 
tion. A  shunt-wound  rotary  tends  to  keep  the  power -factor 
the  same  at  all  loads;  and  in  any  case  the  power-factor  can  be 
adjusted  by  means  of  the  field  rheostat  without  in  any  way 
upsetting  the  regulation.  The  result  of  all  the  complication 
and  disadvantages  of  a  compound-wound  rotary  with  reactance 
coils  is,  that,  often  after  the  system  has  been  in  operation  for 


SHUNT  AND  COMPOUND  WOUND    ROTARIES          73 

some  time,  the  series  magnet-coils  and  the  reactance-coils  are 
cut  out  and  the  rotary  operated  as  a  straight  shunt  machine. 
It  is  then  more  under  the  control  of  the  operator  and  less  liable 
to  give  trouble. 

Most  Satisfactory  System. — Probably  the  best  system  for  gen- 
eral work  is  to  a  have  shunt- wound  rotary,  standard  transform- 
ers, and  no  reactance-coils.  Somewhat  over  excite  the  rotary  so 
as  to  keep  the  power-factor  a  leading  one  at  all  times,  and  then 
leave  the  machines  to  take  care  of  themselves,  only  adjusting 
the  excitation  in  case  they  fail  to  divide  the  load  properly. 

In  the  case  of  a  large  system  the  feeder  drop  is  small  and 
the  fluctuations  in  load  are  unimportant,  so  that  the  voltage 
is  fairly  constant  at  all  times.  The  only  work  then  for  the 
sub-station  attendant  is  to  cut  in  or  out  an  extra  rotary  as  the 
load  requires  it,  and  to  see  that  each  machine  in  circuit  carries 
approximately  its  proper  proportion  of  the  load. 

In  the  case  of  interior  ban  systems  operating  several  small 
sub-stations,  the  voltage  will  vary  with  the  load  on  account  of 
the  feeder  drop.  The  direct  current  voltage  will  be  high  on 
light  loads  and  will  fall  on  full  load.  For  a  feeder  line  with 
sub-stations  at  various  points  this  is  an  ideal  condition.  When 
the  load  is  light  on  any  sub-station  it  will  mean  that  most  of  the 
load  on  the  system  is  being  carried  by  other  stations.  The 
voltage  will  then  be  high  at  the  lightly  loaded  stations  and  lower 
at  the  more  heavily  loaded  ones,  so  that  the  lightly  loaded 
stations  will  tend  to  help  out  the  heavily  loaded  ones,  resulting 
in  ideal  conditions: — a  tendency  to  distribute  the  load  propor- 
tionately between  the  stations  at  all  times.  The  rotaries  should 
be  excited  so  as  to  obtain  either  unity  power  factor  or  a  leading 
one  at  all  loads,  and  then  the  feeder  drop  will  automatically 
take  care  of  the  distribution  of  load,  and  there  will  be  tendency 
always  to  divide  the  load  proportionately  between  all  the  sub- 
stations. 

Thus  with  shunt-wound  rotaries  instead  of  compound-wound 
machines  with  reactance-coils,  there  results  a  cheaper,  more 
efficient,  and  less  complicated  outfit,  one  less  liable  to  give  trouble 


74         SHUNT  AND  COMPOUND   WOUND    ROTAEIES 

and  one  which  will  give  better  results  both  in  large  and  in  small 
stations.  It  is  hardly  to  be  wondered  at  then  that  there  is  at 
the  present  time  a  tendency  to  make  rotaries  shunt- wound; 
and  it  is  probable  the  time  is  not  far  distant  when  the  compound- 
wound  rotary  will  be  considered  as  a  type  to  be  used  only  to 
meet  special  requirements,  the  shunt-wound  rotary  being  ac- 
cepted as  standard  for  all  railway  work. 


[Presented  before  the  American  Institute  of 
Electrical  Engineers,  February  14,  1908.] 


THE  NON-SYNCHRONOUS  OR  INDUCTION  GENERATOR 
IN  CENTRAL  STATION  AND  OTHER  WORK 


THAT  an  induction  motor  could  act  as  a  generator  and  return 
power  to  the  line  when  driven  at  a  speed  above  that  of  synchro- 
nism, has  been  known  for  many  years.  It  has,  however,  always 
been  regarded  as  more  or  less  of  a  scientific  curiosity,  and  except 
in  the  case  of  the  Swiss  three  phase  mountain  railways,  where  the 
motors  are  sometimes  allowed  to  run  as  generators  to  brake  the 
train  on  descending  heavy  grades,  the  induction  generator  has 
had  but  few  commercial  applications.  The  fact  that  the 
characteristics  of  this  generator  are  such  that  it  must  receive 
a  lagging  current  from  the  system,  the  magnitude  of  which  is 
for  a  given  machine  definitely  decided  by  the  slip  of  the  generator 
above  synchronism,  combined  with  the  fact  that  when  connected 
to  a  circuit  it  has  no  definite  voltage  or  frequency  of  its  own, 
make  it  lack  the  flexibility  of  the  synchronous  generator.  In 
1895  it  was  proposed  to  operate  an  induction  generator  in  con- 
junction with  an  unloaded  synchronous  motor,  the  generator 
to  supply  the  watt  component  and  the  motor  the  wattless  com- 
ponent of  the  current  in  the  system.  But  though  this  suggestion 
caused  the  induction  generator  to  become  a  practical  machine, 
it  is  easily  understood  that,  on  account  of  the  lesser  flexibility 
when  compared  with  the  synchronous  generator,  it  has  not 
appealed  to  the  central  station  engineer  as  a  desirable  addition 
to  his  equipment. 

The  question  as  to  the  advisability  of  adopting  the  induction 

75 


76  THE  NON-SYNCHRONOUS  GENERATOR 

generator  for  power  station  work,  was  decided  adversely  by 
engineers  at  the  time  when  the  steam  engine  and  the  water 
turbine  were  the  only  practical  prime-movers.  But  to-day  we 
have  to  deal  with  the  steam  turbine  and  the  gas  engine,  and  the 
introduction  of  these  two  new  types  of  prime-mover  has  al- 
together changed  the  situation  in  regard  to  the  use  of  this 
generator  for  power  station  work.  It  often  happens  in  such 
cases  that  the  real  meaning  and  possibilities  of  the  introduction 
of  types  of  machinery  with  such  fundamentally  new  characteris- 
tics as  the  steam  turbine  and  gas  engine,  are  not  recognized  until 
they  are  accidentally  forced  upon  us.  This  has  been  the  case  in 
the  present  instance;  the  question  of  the  use  of  the  induction 
generator  with  steam  turbines  was  not  seriously  considered  until 
it  was  brought  up  indirectly. 

In  1904  the  Baltimore  Copper  Smelting  and  Rolling  Company 
was  installing  a  1,200  K.W.,  200  volt,  direct  current  generator  for 
electrolytic  work.  It  was  desirable  to  have  the  good  steam 
economy  on  variable  loads,  the  small  floor  space  and  reduced 
maintenance  of  the  steam  turbine,  and  at  the  same  time  to  gen- 
erate 200  volt  direct  current.  A  1,200  K.W.,  6,000  ampere,  200 
volt,  1,800  R.P.M.,  direct  current  turbine  generator  was  not 
considered  practical,  and  an  induction  generator  together  with 
a  rotary  converter  was  suggested  as  an  alternative.  It  was 
decided  to  adopt  this  type  of  equipment,  and  a  1,200  K.W., 
six-phase,  140  volt,  30  cycle,  1,800  R.P.M.  generator  together 
with  a  1,200  K.W.,  six-phase,  150  R.P.M.  rotary  was  installed 
to  supply  the  required  6,000  amperes  direct  current  at  200  volts. 
A  direct  current  exciter  for  the  rotary  was  provided,  so  that  the 
direct  current  voltage  could  be  varied  from  100  to  230  volts  with- 
out any  danger  of  instability.  The  exciter  is  compound  wound 
to  give  constant  voltage  on  the  direct  current  side  of  the  rotary, 
and  the  power-factor  gradually  rises  from  about  25  per  cent  at 
no  load,  to  about  96  per  cent  at  full  load.  When  starting  up 
the  set,  the  rotary  is  run  up  to  speed  from  an  auxiliary  source 
of  direct  current,  and  the  generator  by  its  turbine.  They  are 
then  thrown  together,  the  generator  driving  the  rotary  as  a 


THE  NON-SYNCHRONOUS  GENERATOR  77 

synchronous  motor,  and  the  rotary  supplying  the  magnetizing 
current  of  the  generator.  The  governor  of  the  turbine  decides 
the  frequency  of  the  set,  and  the  slip  of  the  rotary  behind  the 
generator  is  proportional  to  the  load,  being  about  1  per  cent  at 
full  load.  This  equipment  has  been  running  now  for  about 
three  years  and  operates  perfectly.  The  generator  is  similar 
in  arrangement  to  the  old  open-type  turbo-generator,  and  in 
consequence  is  rather  noisy,  but  with  the  modern  enclosed 
type  of  generator,  the  air  circulation  could  be  better  arranged, 
resulting  in  a  quieter  running  machine. 

This  installation  is  given  in  detail,  as  it  is  one  instance  where 
the  adoption  of  this  apparently  inflexible  type  of  generator 
resulted  in  an  installation,  which  combines  the  flexibility  of  the 
standard  direct  or  alternating  current  generator,  with  the  heavy 
overload  capacity  of  the  rotary  converter,  and  the  reliability  and 
robustness  of  construction  of  the  induction  generator.  No  other 
electrical  equipment  which  could  have  been  installed  would 
have  given  the  good  results  that  were  obtained.  And  though 
the  existing  conditions  rather  forced  the  choice  of  the  equip- 
ment in  this  case,  the  result  so  thoroughly  met  expectations, 
that  engineers  began  to  consider  whether  the  development  of 
the  last  few  years  in  regard  to  prime-movers,  had  not  changed 
the  induction  generator  from  a  scientific  curiosity  to  a  machine 
offering  great  advantages  for  certain  conditions  of  power  station 
work. 

General  Characteristics  of  the  Induction  Generator. — As  most 
engineers  are  more  familiar  with  the  characteristics  and  per- 
formance of  the  induction  motor  than  they  are  with  those  of 
the  commercial  induction  generator,  it  will  be  well  to  show  how 
the  characteristics  of  the  two  machines  are  allied.  An  induction 
motor  of  given  characteristics,  carrying  a  certain  definite  load, 
runs  at  a  speed  fixed  relatively  to  that  of  synchronism,  and  it 
takes  a  current  the  magnitude  and  phase  of  which  are  definite. 
The  torque  exerted  by  the  motor  is  proportional  to  the  product 
of  the  current  induced  in  the  short-circuited  secondary,  and  the 
magnetic  flux.  The  electromotive  force  and  the  current  induced 


78  THE  NON-SYNCHRONOUS  GENERATOR 

in  the  secondary,  and  hence  the  torque,  are  proportional  to  the 
rate  at  which  the  secondary  conductors  cut  the  primary  magnetic 
field,  that  is,  to  the  slip  of  the  rotor  above  or  below  the  speed  of 
synchronism.  As  the  speed  of  the  motor  gradually  rises  to 
that  of  synchronism,  the  current  in  the  secondary,  and  the 
torque  gradually  fall,  till  at  the  speed  of  synchronism  they  both 
become  zero.  If  the  speed  of  the  machine  still  continues  to 
increase,  the  secondary  conductors  cut  the  primary  flux  in  the 
reverse  direction,  so  that  the  induced  electromotive  force,  the 
secondary  current,  and  the  torque  become  negative;  that  is,  the 
machine  requires  mechanical  power  to  drive  it  above  the  speed  of 
synchronism.  The  machine  now  returns  electrical  power  to  the 
circuit,  and  has  become  a  generator.  When  running  as  a  motor, 
the  current  is  never  in  phase  with  the  impressed  voltage,  for  two 
reasons:  (1)  the  motor  requires  a  certain  wattless  magnet- 
izing current;  and  (2)  the  motor  windings,  both  primary  and 
secondary,  have  a  certain  amount  of  self-induction.  This  makes 
the  current  lag  behind  the  impressed  voltage  by  an  angle  $, 
depending  on  the  characteristics  of  the  machine,  and  on  the  load 
it  is  carrying.  When  the  machine  runs  above  synchronism  as 
a  generator,  it  still  takes  from  the  circuit  the  wattless  magnet- 
izing current,  and  the  circuits  still  possess  their  self-induction. 
The  result  of  this  is,  that  as  the  watt  component  has  reversed 
in  direction,  the  primary  current  lags  less  than  180°  behind  the 
voltage  impressed  by  the  circuit  on  the  generator,  hence  the 
current  leads  the  electromotive  force  supplied  by  the  generator 
to  the  circuit.  Thus  we  have  as  a  fundamental  characteristic 
of  the  non-synchronous  generator,  that  for  a  given  load,  it  runs 
at  a  certain  definite  speed  above  that  of  synchronism,  and  that, 
(a)  it  supplies  a  watt  current  which  represents  the  power  de- 
livered by  the  generator  to  the  circuit,  and  (b)  it  takes  a  wattless 
magnetizing  current  from  the  system,  the  magnitude  of  which 
depends  on  the  voltage,  and  on  the  watt  component  of  the 
current.  Hence  this  type  of  generator  cannot  supply  a  lagging 
current  to  the  outside  circuit,  and  can  only  deliver  power  to  a 
circuit  which  is  able  to  provide  the  lagging  magnetizing  current 


THE  NON-SYNCHRONOUS  GENERATOR  79 

required  by  an  induction  generator:  while  for  any  given  speed, 
the  magnitude  and  phase  of  the  current  which  the  generator 
will  supply  is  definitely  fixed.  Also,  as  the  wattless  component  of 
the  current  varies  in  magnitude  when  the  load  of  the  machine 
changes,  we  must  have  in  circuit  some  apparatus  which  can  sup- 
ply a  variable  amount  of  lagging  current,  and  which  will  main- 
tain the  voltage  of  the  circuit  constant.  It  is  these  apparently 
rigid  and  inflexible  conditions  that  have  prevented  any  exten- 
sive use  of  the  induction  generator,  and  it  is  only  because  under 
certain  modern  conditions  these  limitations  cease  to  be  serious 
disadvantages,  that  this  generator  is  now  put  forward  as  an 
important  part  of  a  power  station  equipment. 

Usually  the  load  on  a  power  station  is  either  non-inductive, 
or  has  a  lagging  power-factor,  so  that  if  this  type  of  generator 
is  used,  the  lagging  current  required  by  the  generator,  and  per- 
haps also  by  the  outside  circuit,  must  be  artificially  supplied. 
There  are  two  ways  of  obtaining  the  lagging  current  required 
by  the  induction  generator:  (1)  from  a  condenser;  (2)  from  a 
synchronous  generator,  or  an  over-excited  synchronous  motor. 
It  would  not  be  desirable  commercially  to  install  a  condenser 
specially  to  supply  the  required  lagging  current,  as  the  cost 
would  be  prohibitive;  but  a  large  cable  system  has  a  consider- 
able electrostatic  capacity,  and  the  lagging  current  supplied 
by  this  system  will  usually  greatly  reduce  the  size  of  the  neces- 
sary synchronous  machine.  In  any  case,  however,  it  is  necessary 
to  have  a  synchronous  machine,  either  a  motor  or  a  generator, 
in  the  circuit,  in  order  to  set  the  frequency  and  the  voltage.  If 
we  have  the  induction  generator  operating  in  parallel  with  a 
synchronous  unit,  the  latter  machine  supplies  all  the  lagging 
wattless  current  required  by  both  the  induction  generator  and 
the  outside  circuit,  while  the  voltage  of  the  circuit  is  decided  by 
the  excitation  of  the  synchronous  machine.  The  distribution 
of  the  watt  component  of  the  current  between  the  two  machines 
is  decided  by  the  adjustment  of  the  governors  on  the  prime- 
movers.  The  load  which  the  induction  generator  takes,  de- 
pends on  the  percentage  slip  by  which  it  leads  the  synchronous 


80  THE  NON-SYNCHRONOUS  GENERATOR 

unit,  while  the  remainder  of  the  load  is  taken  by  this  latter 
machine.  When  additional  load  comes  on  the  station,  it  comes 
first  on  the  synchronous  generator,  and  then,  as  this  machine 
slows  down  and  allows  the  slip  of  the  induction  generator  to 
increase,  part  of  the  load  is  transferred  to  this  latter  machine. 
In  any  case,  the  voltage  regulation  of  the  system  is  that  of 
the  synchronous  unit,  and  the  voltage  of  the  circuit  under 
any  condition  of  load  is  decided  by  the  excitation  of  this 
machine. 

If  the  induction  generator  is  operating  in  parallel  with  a 
synchronous  motor  or  rotary  converter,  the  same  remarks  apply. 
The  voltage  regulation  is  that  of  the  synchronous  machine,  and 
the  voltage  of  the  circuit  is  decided  by  the  magnitude  of  its 
excitation.  When  the  energy  load  on  the  two  machines  increases, 
the  additional  load  comes  first  on  the  synchronous  machine, 
energy  being  supplied  to  the  system  from  the  momentum  of  its 
rotating  part;  and  then  as  the  machine  slows  down  a  little,  the 
load  is  transferred  to  the  induction  generator,  the  synchronous 
machine  supplying  only  any  increase  in  the  wattless  component 
of  the  external  load,  together  with  the  additional  lagging  current 
required  by  the  generator  when  carrying  the  increased  energy 
load.  The  synchronous  machine  supplies  at  all  times  all  the 
lagging  wattless  current  in  the  circuit,  and  the  governor  of  the 
prime-mover  driving  the  induction  generator  decides  the  fre- 
quency of  the  circuit,  the  synchronous  machine  slipping  behind 
the  generator  an  amount  sufficient  to  allow  this  latter  machine 
to  supply  all  the  power  required  by  the  circuit. 

Non-synchronous  Generators  in  Power  Station  Work. — Ob- 
viously the  disadvantage  of  this  type  of  generator  for  power 
station  work  is,  that  it  cannot  supply  a  lagging  wattless  current, 
and  that  it  requires  an  additional  lagging  wattless  current  to 
excite  it.  The  power-factor  of  the  current  supplied  by  such  a 
generator  is  a  direct  measure  of  the  amount  of  wattless  current 
required  to  excite  it  under  that  particular  load.  The  power- 
factor  which  can  be  obtained  in  designing  any  induction  genera- 
tor, depends  on  the  size,  speed,  voltage,  and  frequency  of  the 


THE  NON-SYNCHRONOUS  GENERATOR 


81 


machine.     Low  speed,  high  voltage,  and  high  frequency,  all  tend 
to  lower  the  power-factor  which  can  be  obtained. 

Table  I  gives  the  characteristics  of  steam  turbine  and  gas 
engine  driven  induction  generators  of  from  1,000  K.W.  to  10,000 

TABLE  I 

STEAM  TURBINE    DRIVEN    25    CYCLE    INDUCTION    TYPE    GENERATORS 


LOAD. 

Kilo- 
watts. 

Rev. 

per  min. 

Volts. 

No-load 
Cur- 
rent. 

Full 
Load 
Slip. 

1 

0.75 

0.50 

1 

0.75 

0.50 

Efficiency. 

Power-factor. 

1000 

1500 

2200 

97.6 

97.7 

97.5 

97.0 

97.5 

97.0 

8.3% 

0.75% 

13000 

95.0 

95.9 

96.5 

2500 

1500 

2200 

98.2 

98.2 

97.9 

97.9 

97.3 

96.4 

8.3 

0.48 

13000 

96.5 

96.9 

96.0 

5000 

1500 

2200 

98.3 

98.2 

98.0 

98.0 

97.7 

97.4 

8.5 

0.46 

13000 

96.5 

97.0 

96.5 

10000 

1500 

2200 

98.5 

98.4 

98.2 

98.2 

98.0 

97.5 

8.1 

0.40 

13000 

96.8 

97.3 

97.1 

60  CYCLE 


1000 

1800 

2200 

97.6 

97.7 

97.5 

96.4 

96.9 

96.4 

9.5 

0.75 

13000 

94.0 

95.0 

94.5 

115 

2500 

1800 

2200 

98.2 

98.2 

97.9 

97.2 

96.8 

96.2 

9.5 

0.48 

13000 

94.5 

95.3 

'.'1  s 

5000 

1200 

2200 

98.3 

98.2 

98.0 

96.0 

97.2 

97.0 

9.5 

0.45 

13000 

94.5 

95.6 

95.5 

10000 

1200 

2200 

98.5 

98.4 

98.2 

97.6 

97.6 

97.1 

9.5 

0.40 

13000 

95.3 

95.6 

95.5 

GAS  ENGINE   DRIVEN  25  CYCLE  INDUCTION  TYPE  GENERATORS 


1000 

94 

2200 

96.7 

97.1 

97.2 

94.0 

94.2 

92.0 

16.5 

1.5 

13000 

89.3 

90.9 

88.7 

18.0 

2000 

83 

2200 

97.0 

97.4 

97.6 

95.5 

95.1 

94.0 

16.5 

1.4 

13000 

92.4 

93.2 

90.7 

3500 

75 

2200 

97.1 

97.5 

97.7 

95.7 

95.2 

94.2 

16.5 

1.4 

13000 

92.6 

93.4 

91.0 

60  CYCLE 


1000 

93 

2200 

95.5 

95.7 

96.0 

88.8 

88.5 

84.6 

25.0 

1.8 

13000 

83.0 

81.0 

73.3 

40.0 

2000 

82 

2200 

96.0 

96.3 

96.5 

89.5 

89.2 

85.6 

24.0 

1.7 

13000 

83.5 

81.8 

75.0 

38.0 

3500 

75 

2200 

96.3 

96.5 

96.7 

90.8 

89.5 

87.0 

22.5 

1.6 

13000 

85.0 

83.3 

77.0 

35.0 

K.W.  for  2,200  or  13,200  volts,  and  for  25  or  60  cycles.  The 
table  shows  that  on  high  speed  2,200  volt  generators,  the  power 
factor  rises  as  high  as  98.25  per  cent,  and  that  we  can  obtain  on 

6 


82  THE   NON-SYNCHRONOUS  GENERATOR 

such  machines  a  power-factor  which  averages  97  per  cent  to 
98  per  cent,  from  one-half  to  one  and  one-quarter  load ;  while  the 
no-load  magnetizing  current  is  less  than  10  per  cent  of  the  full 
load  current  of  the  machine.  This  being  the  case,  the  fact  that 
these  generators  require  a  wattless  current  to  excite .  them 
ceases  to  be  a  serious  objection,  and  it  would  appear  that  the 
only  important  limitation  of  this  type  of  machine  is,  that  it 
cannot  supply  lagging  wattless  current  to  the  outside  circuit. 
The  low-speed,  60  cycle  machines  are  relatively  poor  as  regards 
power-factor  and  exciting  current,  so  that  the  use  of  induction 
generators  would  not  be  advocated  under  these  conditions, 
unless  their  other  characteristics  made  them  particularly  ad- 
vantageous for  the  special  conditions  under  which  they  were  to 
be  used.  It  will  also  be  seen  from  Table  I  that  another  advan- 
tage of  this  type  of  generator  is  the  extremely  high  efficiency  at 
all  loads  that  is  obtained  in  high-speed  machines.  As  a  result  of 
this,  these  generators  have  a  low  temperature  rise  at  normal 
rated  load,  and  have  a  large  overload  capacity.  The  normal 
ratings  of  the  individual  machines  given  in  the  table  were  chosen 
so  that  their  characteristics  would  be  best  at  from  one-half  to  one 
and  one-quarter  rated  load.  All  the  machines  given  can  gen- 
erate from  two  and  one-half  to  five  times  their  rated  output,  and 
as  far  as  general  characteristics  are  concerned  could  be  rated  50 
per  cent  higher.  It  was  stated  above  that  the  slip  of  the  in- 
duction generator  relatively  to  the  synchronous  unit  must  in- 
crease from  no  load  to  full  load,  in  order  that  the  former  may 
carry  its  due  share  of  the  load.  So  if  it  is  required  that  the  load 
always  be  automatically  divided  between  the  two  machines, 
we  must  adjust  the  governor  of  the  synchronous  prime-mover, 
so  that  its  drop  in  speed  from  no  load  to  full  load  is  equal  to 
the  drop  in  speed  of  the  non-synchronous  prime-mover  plus  the 
full  load  slip  of  the  non-synchronous  generator.  This  means 
that  the  speed  and  frequency  of  the  synchronous  generator  must 
change  with  the  load,  but  we  see  from  the  slip  given  in  the  table 
that  this  change  is  unimportant.  The  slip  varies  from  0.4  per 
cent  to  0.75  per  cent  in  high-speed  machines,  and  from  1.4  per 


THE   NON-SYNCHRONOUS  GENERATOR  83 

cent  to  1.8  per  cent  in  low-speed  machines,  and  this  is  about  as  close 
as  the  governors  can  be  made  to  regulate  in  any  prime-mover. 

The  greatest  commercial  field  for  the  induction  generator 
is  undoubtedly  in  connection  with  steam  turbine  driven  genera- 
tors. This  type  of  generator  is  more  suitable  both  mechanically 
and  electrically  for  high  speed  work  than  any  other  type  of  elec- 
trical machine.  The  squirrel  cage  secondary  with  heavy  copper 
bars,  each  bar  held  in  a  separate  closed  slot,  and  practically 
requiring  no  insulation,  is  an  ideal  construction  mechanically, 
and  is  one  which  can  be  operated  at  very  high  temperature 
without  damage.  Comparing  this  with  the  rotating  magnets 
of  the  standard  synchronous  turbo  alternator,  the  difference  is 
very  great.  The  magnet  winding  of  a  synchronous  turbo  alter- 
nator consists  of  a  number  of  turns  of  thin  strap  separated  by 
insulation;  while  the  windings  often  reach  high  temperatures 
due  to  over  load  at  low-power  factors,  and  are  in  addition  subject 
to  heavy  centrifugal  stresses  and  a  potential  difference  of  125 
volts  to  ground.  We  can  see  that  such  a  construction  does  not 
compare  favorably  with  that  of  the  squirrel  cage  rotating  sec- 
ondary of  an  induction  generator;  and  as  a  breakdown  on  the 
field  of  a  synchronous  turbo  generator  usually  puts  the  machine 
out  of  commission  for  a  couple  of  weeks,  the  commercial  advan- 
tages possessed  by  the  non-synchronous  generator  are  obvious. 

The  simplicity  in  construction  and  insulation  of  the  rotating 
parts,  the  ease  with  which  the  centrifugal  stresses  necessarily 
present  can  be  taken  care  of,  and  the  absence  of  a  complicated 
winding  or  brush  gear,  obviously  tend  to  reduce  the  cost  of  the 
induction  generator  compared  with  that  of  the  standard  syn- 
chronous generator.  The  actual  cost  of  any  machine,  as  dis- 
tinguished from  the  sale  price,  depends  on  the  performance 
specification  to  which  it  is  designed,  and  on  how  closely  it 
is  rated,  but  it  can  readily  be  seen  that  the  induction 
generator  offers  facilities  for  cheaper  design  and  manufacture 
which  are  not  presented  by  the  synchronous  generator. 

The  Excitation  of  the  Induction  Generator. — A  synchronous 
generator  requires  direct  current  excitation,  while  an  induction 


84  THE  NON-SYNCHRONOUS  GENERATOR 

generator  requires  alternating  current  for  its  excitation.  The 
induction  generator  is  excited  by  a  lagging  current  taken  usually 
from  a  synchronous  machine,  and  as  this  synchronous  unit 
requires  direct  current  excitation  to  produce  this  lagging  current, 
it  can  be  said  that  indirectly  the  induction  generator  does  require 
a  direct  current  exciter.  But  on  account  of  the  small  air  gap 
of  this  type  of  generator,  it  requires  much  less  excitation  than 
a  synchronous  machine.  The  actual  capacity  of  the  exciters 
required  by  a  power  station  consisting  of  induction  generators, 
depends  on  the  power-factor  of  the  load  on  the  system.  The 
capacity  will  usually  vary  from  one-quarter  to  one-half  of  that 
which  would  be  required  for  the  corresponding  synchronous 
generators;  though  if  the  power  station  feeds  a  cable  system 
with  high  electrostatic  capacity,  this  will  supply  part  of  the  re- 
quired lagging  excited  current,  and  reduce  the  size  of  the  required 
exciter.  The  charging  current  of  the  New  York  Interborough 
system  is  given  as  about  105  amperes  at  11,000  volts,  i.e.,  about 
2,000  K.V.A.;  and  that  of  the  New  York  Edison  system  is  about 
40  amperes  at  6,600  volts,  i.e.,  about  450  K.V.A.  From  Table 
I  it  is  evident  that  the  capacity  charging  current  of  the  New 
York  Edison  system  is  sufficient  to  supply  full  load  magnetizing 
and  wattless  current  required  by  a  2,000  K.W.,  6,600  volt,  25 
cycle  turbine  driven  non-synchronous  generator,  when  running 
on  a  non-inductive  external  load.  In  the  same  way  the  capacity 
charging  current  of  the  Interborough  system  would  be  sufficient 
to  supply  the  wattless  current  of  a  10,000  K.W.,  11,000  volt,  25 
cycle  turbine  driven  generator.  If  we  had  a  cable  system,  such 
as  the  Interborough,  distributing  at  20,000  volts,  it  would  have 
a  charging  current  of  190  amperes  at  20,000  volts,  which  would 
be  sufficient  to  supply  the  wattless  component  for  40,000  K.W. 
in  22,000  volt,  25  cycle  turbine  driven  non-synchronous  genera- 
tors. So  we  can  see  that  the  electrostatic  capacity  of  a  large 
cable  system  will  play  an  important  part,  when  the  introduction 
of  such  generators  into  some  of  the  large  New  York  power 
stations  is  considered. 

In  a  system  consisting  of  induction  generators  supplying 


THE  NON-SYNCHRONOUS  GENERATOR  85 

power  to  rotary  converters,  it  is  unnecessary  to  have  any  ex- 
citers or  synchronous  units  in  the  power  station.  The  first 
rotary  started  up,  must  be  brought  up  to  speed  from  the 
direct  current  side  and  then  thrown  on  the  generator  circuit, 
when  it  will  excite  the  latter,  the  voltage  being  decided  by  the 
excitation  of  the  rotary.  We  can  see  from  Table  I  that  the 
power-factor  of  an  induction  generator  can  be  made  to  remain 
practically  constant  from  one-half  to  one  and  one-quarter  load, 
so  that  the  amount  of  wattless  current  taken  by  the  generator 
throughout  its  normal  working  range  will  be  practically  pro- 
portional to  the  watt  component  of  the  current.  Hence,  assum- 
ing we  can  neglect  the  capacity  charging  current  of  the  cable 
system;  if  we  have  a  number  of  rotaries  operating  on  the  circuit, 
we  can  adjust  the  shunt  excitation  so  as  to  give  the  correct  volt- 
age at  no  load,  while  the  series  excitation  can  be  adjusted  to 
obtain  any  desired  voltage  characteristic  as  the  load  comes  on 
the  system,  the  rotaries  compounding  the  generators  by  their 
series  winding.  This  compounding  of  the  generators  as  the  load 
comes  on  any  sub-station,  affects  of  course  all  the  other  sub- 
stations fed  from  the  generators;  so  if  we  are  not  regulating  for 
constant  voltage,  it  may  be  advisable  in  some  cases  to  introduce 
artificial  self-induction  into  the  rotary  feeder  circuits,  so  as  to 
over  compound  these  circuits  rather  than  the  generators,  and 
avoid  disturbing  the  voltage  on  other  unloaded  sub-stations. 
In  a  large  system,  the  capacity  current  of  the  cables  cannot  be 
neglected,  so  the  wattless  current  to  be  supplied  by  the  rotaries 
will  not  be  directly  proportional  to  the  load.  Such  a  system, 
however,  usually  requires  constant  voltage  at  the  direct  current 
terminals,  and  when  such  is  the  case  it  may  be  found  advan- 
tageous to  install  compound-wound  rotaries  with  automatic  volt- 
age regulators  to  control  the  shunt  excitation.  The  voltage 
regulators  could  then  be  made  to  keep  the  generator  voltage 
constant,  while  the  series  winding  would  serve  to  compound 
each  individual  feeder  in  order  to  compensate  for  the  voltage 
drop  in  that  feeder.  In  such  a  system,  all  regulators  controlling 
the  voltage  on  a  given  group  of  generators  would  be  tied  together, 


86  THE  NON-SYNCHRONOUS  GENERATOR 

so  that  any  one  regulator  could  not  act  before,  or  act  against  the 
others,  and  make  the  shunt  excitation  converters  different  for 
the  individual  rotaries. 

Assuming  the  self-induction  is  negligible,  then  if  there  is  no 
electrostatic  capacity  in  the  system,  the  power-factor  of  the 
rotaries  is  practically  the  same  as  that  of  the  generators;  but 
if  there  is  capacity  which  helps  to  supply  the  lagging  current, 
then  the  power-factor  of  the  rotaries  will  be  higher  than  that  of 
the  generators.  Taking  once  more  the  Interborough  system, 
and  assuming  there  are  75,000  K.W.  of  11,000  volt  turbine- 
driven  non-synchronous  generators  in  the  power  station,  and 
75,000  K.W.  of  rotary  converters  in  the  sub-station,  then  the 
capacity  current  supplies  13  per  cent  of  the  wattless  current 
taken  by  the  generator  on  full  load,  and  the  full  load  power- 
factor  for  the  rotaries  will  be  98  per  cent.  Such  a  power  station 
of  induction  generators,  having  no  direct  current  exciters  and 
exciting  circuits,  is  much  simpler  as  regards  cables  and  switch- 
board connections  than  a  similar  station  with  synchronous 
generators,  and  is  much  simpler  to  operate.  There  is  no  neces- 
sity for  synchronizing  the  generators;  they  are  run  up  to  speed 
and  thrown  on  the  line  in  series  with  a  choke  coil  (to  limit  the 
rush  of  current);  the  choke  coil  being  then  short-circuited,  the 
generators  are  automatically  excited  from  the  rotaries  and  take 
care  of  themselves.  The  governors  of  the  prime-movers  are 
controlled  by  pilot  motors  from  the  switchboard,  and  the  load 
can  be  distributed  at  will  among  the  different  generators  without 
any  adjustment  of  the  excitation  to  keep  the  power-factor  con- 
stant, as  would  be  necessary  with  synchronous  generators. 
This  results  in  an  ideally  simple  station,  as  there  are  no  auxiliary 
circuits,  and  the  switchboard  is  practically  limited  to  the  main 
generator  and  feeder  switches  and  instruments. 

Short  Circuits  and  Resonance. — During  the  last  few  years 
we  have  heard  a  great  deal  regarding  resonance  and  high  power 
surges  in  large  installations  with  cable  distributing  systems  of 
high  electrostatic  capacity.  These  phenomena  can  be  investi- 
gated mathematically  in  detail,  if  we  choose  to  make  a  number 


THE  NON-SYNCHRONOUS  GENERATOR  87 

of  more  or  less  arbitrary  assumptions;  but  on  account  of  the 
arbitrariness  of  these  assumptions,  we  are,  from  the  practical 
engineer's  standpoint,  justified  at  the  present  time  in  describ- 
ing these  phenomena  only  in  general  terms.  By  a  high  power 
surge,  is  meant  the  oscillation  sometimes  set  up  in  a  system  by 
a  sudden  rush  of  current,  such  as  a  short-circuit,  and  which  has 
usually  the  fundamental  frequency  of  the  circuit.  The  power 
represented  by  such  a  surge  is  proportional  to  the  square  of  the 
value  reached  by  the  current  in  the  first  sudden  rush,  while  the 
rise  in  voltage  is  directly  proportional  to  the  current  surge. 
Resonance  effects  cover  the  extremely  high  rises  of  potential 
which  take  place  in  a  circuit  containing  self-induction  and 
capacity,  when  the  frequency  of  the  circuit  has  a  certain  critical 
value  dependent  on  the  amount  of  induction  and  capacity  in 
circuit.  In  the  usual  power  systems,  resonance  cannot  generally 
take  place  at  the  normal  frequency  of  the  circuit;  but  there 
are  usually  higher  harmonics  of  this  normal  frequency,  intro- 
duced by  distortion  of  the  fundamental  wave-form,  and  there 
may  be  resonance  and  high  voltage  due  to  one  of  these  high 
harmonics. 

This  short  analysis  shows  at  once  why  a  power  station  of 
synchronous  generators  is  so  liable  to  suffer  from  surges  and 
resonance.  Synchronous  generators  and  motors  will  give  a 
greater  sudden  rush  of  current,  or  surge,  in  the  case  of  short- 
circuit  than  almost  any  other  class  of  machine.  And  though 
they  have  voltage  wave  forms  which  approximate  closely  to  a 
sine  wave  on  no  load,  these  wave-forms  become  so  distorted  by 
armature  reaction  on  load,  and  change  so  much  with  the  magni- 
tude and  phase  of  the  current,  that  there  is  an  excellent  chance 
of  introducing  such  harmonics  as  will  produce  resonance.  If 
we  were  deliberately  to  try  to  choose  conditions  which  would 
be  most  liable  to  give  trouble  from  high-power  surges  and 
resonance,  we  could  not  well  choose  anything  that  would  be 
worse  than  synchronous  generators  feeding  synchronous  motors 
through  a  cable  system  of  high  capacity.  The  induction  gen- 
erator is  a  great  contrast  to  the  synchronous  in  this  respect,  as 


88  THE  NON-SYNCHRONOUS  GENERATOR 

it  tends  rather  to  eliminate  disturbances  from  the  line  than  to 
originate  them.  A  short-circuit  on  a  system  results  in  the 
voltage  falling  to  zero,  consequently  any  induction  generator  on 
the  circuit  losing  its  excitation  becomes  dead,  and  does  not  tend 
to  supply  either  power,  current,  or  voltage  to  the  short-circuit. 
Further,  the  voltage  wave  form  of  an  induction  generator  is 
virtually  a  sine  wave  for  all  loads,  and  has  no  tendency  at  all  to 
introduce  higher  harmonics  which  might  produce  resonance.  If 
the  synchronous  machines  supplying  the  wattless  current  in  the 
circuit  have  a  badly  distorted  wave  form,  the  magnetizing  cur- 
rent of  the  induction  generator  will  also  be  distorted,  but  there 
will  be  a  strong  tendency  to  damp  out  all  harmonics  in  the  volt- 
age wave  form  of  the  system;  and  we  can  say  that,  generally 
speaking,  the  induction  generator  acts  as  a  strong  damper  to 
remove  all  harmonics  in  the  voltage  wave  form  of  the  system, 
introduced  by  distortion  of  the  wave  form  of  any  synchronous 
machines.  This  distortion  in  a  synchronous  machine,  being  due 
to  the  armature  reaction  of  the  watt  component  of  the  current, 
rather  than  the  wattless,  we  see  that  the  best  conditions  as  re- 
gards freedom  from  distortion  and  harmonics  are  obtained  by 
the  use  of  a  rotary  converter  or  unloaded  synchronous  machine, 
rather  than  a  loaded  synchronous  generator  or  motor  to  supply 
the  wattless  current  required  to  excite  an  induction  generator. 
Fig.  24  shows  approximately  the  relation  of  the  watt  compo- 
nent to  the  wattless  component  of  the  current  supplied  by  a 
2,000  K.W.  induction  generator,  the  curve  being  for  its  normal 
rated  voltage  of  11,000  volts;  for  a  different  voltage  the  values 
of  current,  both  watt  and  wattless,  should  be  multiplied  by  the 
ratio  of  the  new  voltage  to  11,000  volts.  We  can  see  from  this, 
that  the  magnitude  of  the  watt  current  bears  a  definite  relation  to 
that  of  the  wattless,  and  that  the  watt  current,  and  consequently 
the  load  on  the  machine,  cannot  change  without  the  wattless  cur- 
rent also  changing.  Further,  for  each  point  on  the  curve,  the 
slip  of  the  induction  generator  ahead  of  the  synchronous  machine 
has  a  certain  definite  value.  This  shows  that  when  a  short- 
circuit  comes  on  a  system  consisting  of  an  induction  generator 


THE  NON-SYNCHRONOUS  GENERATOR 


89 


and  synchronous  generator  or  motor,  the  short-circuit  will  come 
on  the  synchronous  machine.  If  the  voltage  drops  to  zero,  the 
induction  generator  will  be  dead;  but  if  the  short-circuit  is  not 
severe  enough  to  reduce  the  voltage  of  the  system  to  zero,  then 
it  may  still  supply  current  to  the  circuit.  The  amount  which  it 
supplies  will  depend  on  the  way  in  which  the  excitation  of  the 
synchronous  machines  is  changed  by  any  automatic  voltage 
regulators.  But  a  change  in  load  which  can  be  taken  care  of  by 
voltage  regulators  hardly  comes  under  the  class  of  short-circuits, 


Full  Load 


Wattless  current  (curve  A),  and  slip  (curve  B). 
FIG.  24. 

and  as  these  latter  effects  are  the  only  serious  ones,  we  will  con- 
sider them  alone. 

We  see  from  the  above  that  the  induction  generator  takes  no 
part  in  the  sudden  surge  of  current  which  occurs  on  a  short- 
circuit,  so  that  this  surge  cannot  be  greater  than  that  which  is 
supplied  by  the  synchronous  machines  in  circuit.  This  sudden 
surge  which  takes  place  when  any  synchronous  machine,  whether 
generator,  motor,  or  rotary,  is  short-circuited  is  equal  to: 

Electromotive  force  of  synchronous  machine 
Total  impedance  in  circuit 

After  the  current  has  flowed  for  an  appreciable  interval,  so  that 
the  magnetism  of  the  synchronous  machine  has  had  time  to 


90  THE   NON-SYNCHRONOUS  GENERATOR 

change,  the  armature  reaction  cuts  down  the  electromotive 
force  generated,  and  the  current  falls  to  the  value  commonly 
known  as  the  short-circuit  value,  this  being  the  value  of  the 
current  on  a  continuous  short-circuit.  The  5,000  K.W.,  11,000 
volt  Manhattan  generators  will  give  a  continuous  short-circuit 
current  about  three  times  full  load  current,  but  the  instantaneous 
value  of  the  current  on  a  sudden  short-circuit  is  about  seven  times 
this;  that  is,  about  twenty-one  times  normal  full-load  current. 
If  these  generators  are  supplying  rotary  converters  these 
rotaries  will  also  supply  power  to  the  short-circuit.  Operating 
as  generators,  the  1,500  K.W.  Manhattan  converters  give  about 
full-load  current  on  a  continuous  short-circuit  on  the  alternating 
current  side,  with  the  self-induction  of  the  transformers  and  reac- 
tive coils  in  circuit;  and  the  instantaneous  value  on  sudden  short- 
circuit  is  about  three  times  this  value.  So  a  short-circuit  on  a 
system  consisting  of  the  Manhattan  generators  supplying  power 
to  rotary  converters  will  give,  on  sudden  short-circuit,  a  rush  of 
current  equal  to  twenty-four  times  the  total  full-load  current  of 
the  generators  in  circuit;  while  after  a  short  period  the  value  of 
the  short-circuit  current  will  fall  to  about  one-sixth  of  this  value. 
Assuming  that  we  had  induction  instead  of  synchronous  genera- 
tors in  the  power  station,  the  short-circuit  current  would  be 
limited  to  that  from  the  rotaries,  and  the  sudden  surge  would 
be  equal  to  three  times  full-load  current.  The  voltage  of  the 
system  would  fall  to  zero,  and  the  rotaries  would  supply  a 
gradually  decreasing  current  to  the  'short-circuit,  until  their 
rotational  energy  was  expended,  and  they  had  come  to  rest. 
We  see  then  that  with  induction  generators  in  the  power  station, 
the  magnitude  of  the  sudden  surge  on  a  short-circuit  would  be 
reduced  to  one-eighth  of  that  which  would  take  place  with  the 
present  synchronous  generators.  As  a  result  of  this  the  voltage 
rise  would  be  only  one-eighth,  and  the  power  of  the  surge 
would  be  one  sixty-fourth.  These  figures  do  not  need  any 
comment. 

There  is  one  point,  however,  that  must  be  considered  when 
operating  non-synchronous  generators  on  a  system  containing 


THE  NON-SYNCHRONOUS  GENERATOR  91 

considerable  electrostatic  capacity,  and  that  is,  that  the 
individual  generator  units  are  not  too  small.  An  induction 
generator  can  be  excited  by  the  lagging  current  from  a 
condenser,  and  the  voltage  to  which  it  will  be  excited  depends 
on  the  size  of  the  condenser.  In  a  system  consisting  of  induc- 
tion generators  and  synchronous  units,  we  might,  as  the  result 
of  opening  circuit  breakers  by  line  disturbances,  have  one 
generator  and  one  small  synchronous  unit  left  on  the  line. 
The  capacity  current  of  the  cable  system  would  then  tend  to 
build  up  the  voltage  of  the  machines  until  the  saturation  of 
the  magnetic  circuit  prevented  any  further  rise.  Taking  the 
2,000  K.W.,  11,000  volt  induction  generator,  the  current  curves 
of  which  are  shown  in  Fig.  24;  the  magnetizing  current  at 
11,000  volts  is  9  amperes.  If  the  capacity  charging  current 
of  the.  cables  is  100  amperes  at  11,000  volts,  and  the  synchro- 
nous machine  is  so  small  as  to  take  a  negligible  lagging 
magnetizing  current,  the  voltage  of  both  units  would  probably 
build  up  to  double  the  normal.  If,  however,  the  smallest 
machine  on  the  circuit  were  a  10,000  K.W.  generator,  and  a 
1,500  K.W.  synchronous  motor  or  rotary  converter,  the  rise  in 
voltage  would  not  be  more  than  10  per  cent,  while  if  the 
minimum  generator  unit  had  been  20,000  K.W.,  there  would 
be  no  rise  in  voltage;  so  that  this  is  a  condition  which  can  be 
taken  care  of  when  designing  the  station. 

Distortion  of  the  wave  form  introduces  higher  harmonics, 
and  may  cause  resonance  or  cross-currents.  In  a  synchronous 
generator  or  motor,  the  wave  form  of  the  magnetism  is  usually 
badly  distorted  when  operating  under  load,  the  distortion 
being  greater  the  higher  the  power-factor,  and  the  greater  the 
load.  This  distortion  of  the  magnetic  wave  introduces  higher 
harmonics  into  the  wave  form  of  the  electromotive  force  gener- 
ated in  the  armature  conductors;  the  most  important  harmonic 
introduced  being  the  third,  though  the  fifth,  seventh,  ninth, 
and  higher  harmonics  are  also  usually  presented.  In  a  three- 
phase  winding,  the  third  harmonic,  and  also  harmonics  of  this 
third  harmonic,  appear  in  the  electromotive  force  between  the 


92  THE  NON-SYNCHRONOUS  GENERATOR 

neutral  and  outer  terminals,  but  not  in  the  electromotive  force 
between  the  outer  terminals.  They  therefore  appear  in  a  three- 
phase,  four-wire  system,  or  in  a  three-phase  system  with  ground- 
ed neutral.  The  other  harmonics  appear,  no  matter  what  the 
connections  are.  Though  the  presence  of  these  harmonics  may 
not  cause  harm  in  any  individual  case,  it  is  always  possible 
that  there  may,  under  certain  conditions  of  circuit  and  load, 
be  harmonics  of  a  frequency  sufficiently  close  to  that  of  resonance 
to  cause  serious  rise  of  potential.  And  if  generators  of  different 
characteristics  operate  together,  or  if  there  are  loads  of  different 
magnitude  and  phase,  or  different  excitation  on  the  synchronous 
generators  or  motors,  cross-currents  are  liable  to  be  produced 
between  the  machines,  this  being  especially  the  case  when 
running  with  a  grounded  neutral.  Rotary  converters  are  very 
much  better  than  any  other  class  of  synchronous  machines  as 
regards  distortion  of  wave-form  when  operating  with  unity 
power-factor,  as  they  have  practically  no  armature  reaction;  so 
that  the  electromotive  force  wave  generated  under  such  con- 
ditions is  approximately  a  sine.  The  above  remarks  on  syn- 
chronous generators  and  motors,  then,  only  apply  to  rotary 
converters  to  a  limited  extent. 

Induction  generators  have  no  distortion  of  field  due  to 
armature  reaction,  and  as  long  as  the  iron  in  the  magnetic  circuit 
is  not  saturated,  the  electromotive  force  wave-form  of  these 
generators  is  virtually  a  sine  wave  for  all  conditions  of  load. 
In  consequence  of  this  there  can  be  no  cross  currents  between 
generators  of  this  type,  due  to  difference  in  wave-form,  and 
no  tendency  to  produce  resonance  in  the  circuit.  Furthermore, 
an  induction  generator  acts  as  the  strongest  possible  damper  in  a 
circuit;  and  if  there  is  any  surge,  unbalancing  of  phases,  distor- 
tion of  wave-form  or  hunting  present,  the  induction  generator 
will  tend  to  damp  it  out,  and  to  restore  the  original  condition  of 
steady  sine  wave  operation.  If  we  then  have  a  system  con- 
sisting of  induction  generators  supplying  power  to  rotary  con- 
verters, it  would  be  as  nearly  perfect  as  possible  in  its  freedom 
from  surges  and  resonance.  The  rotary  converter  would  give 


THE  NON-SYNCHRONOUS  GENERATOR  93 

the  minimum  distortion  of  any  synchronous  machine,  and  the 
induction  generator  would  tend  to  damp  out  any  disturbance 
that  occurred  on  the  line.  Such  a  system  would  certainly  be 
much  superior  to  any  one  containing  synchronous  generators 
and  synchronous  motors,  in  regard  to  liability  to  disturbances 
from  resonance  or  surges,  and  in  all  probability  the  engineers  of 
such  a  system  operating  with  grounded  neutral  would  not  be 
made  aware  that  such  phenomena  existed. 

I  have  endeavored  to  show  that  the  induction  generator  is 
very  much  superior  to  the  synchronous  from  almost  every  point 
of  view,  for  the  purpose  of  supplying  power  to  motor  generators 
or  rotary  converters  through  an  underground  cable  system;  and 
that  rotary  converters  are  less  liable  to  introduce  line  disturb- 
ances than  synchronous  motors.  In  some  cases  it  might  be 
considered  advisable  to  install  both  synchronous  and  induction 
generators,  or  the  station  might  be  one  in  which  the  units  first 
installed  were  synchronous  while  the  later  extensions  were 
induction  generators.  In  such  cases  it  is  readily  seen  that  the 
advantages  as  outlined  above,  are  obtained  to  a  degree  which 
depends  on  the  ratio  of  the  capacity  in  induction  units  to  those 
in  synchronous.  It  should  be  remembered  that  the  induction 
generator  and  rotary  converter  give  the  best  combination  to 
insure  freedom  from  line  disturbances,  and  that  the  synchronous 
generator  and  synchronous  motor  give  the  worst;  combinations 
of  the  two  systems  give  results  intermediate  between  these 
two. 

Small  Power  Stations. — Above  we  have  considered  in  detail 
large  systems  which  can  easily  provide  the  necessary  lagging 
exciting  current  for  the  induction  generator,  either  from  the 
charging  current  of  a  cable  system  or  from  synchronous  motors 
or  rotary  converters  in  the  system.  With  smaller  stations 
which  supply  power  direct  to  motor  and  lighting  circuits,  the 
conditions  are  not  so  favorable  to  the  induction  generator,  as 
this  type  is  primarily  one  for  high  power-factor  loads,  and  is  at 
a  distinct  disadvantage  in  a  station  where  the  load  is  of  low 
power-factor.  The  advantages  of  the  non-synchronous  genera- 


94  THE  NON-SYNCHRONOUS  GENERATOR 

tor  as  outlined  above  are.  however,  so  great,  that  each  particular 
case  should  be  considered  in  detail  to  determine  whether  its 
adoption  is  advisable.  Usually  the  low  power-factor  load  on 
such  a  station  consists  mainly  of  motors  operating  only  during 
the  daytime,  whereas  the  heavy  peak  load  is  the  lighting  load  at 
night.  We  can  therefore  install  synchronous  generators  sufficient 
to  carry  the  low  power-factor  day  motor  load,  and  induction 
generators  to  assist  in  carrying  the  lighting  load  at  night.  Con- 
sidering a  60  cycle  station  having  a  day  load  of  2,500  K.V.A., 
with  a  power-factor  of  70  per  cent,  and  a  night  load  of  4,000 
K.V.A.,  with  a  power-factor  of  95  per  cent;  and  assume 
further  that  we  have,  exclusive  of  spares,  two  1,250  K.V.A. 
synchronous  turbo  generators  to  carry  the  day  load,  and  two 
1,250  K.W.  induction  generators  to  assist  in  carrying  the 
night  load.  This  night  load  consists  of  3,800  watt  K.V.A. 
and  1,250  wattless  K.V.A.,  while  the  two  induction  generators 
require  in  addition  640  K.V.A.  to  excite  them.  We  shall 
then  have  the  two  synchronous  units  carrying  a  load  of 
2,290  K.V.A.  at  52  per  cent  power-factor  and  the  non-syn- 
chronous machines  carrying  a  load  of  2,500  K.W.  at  power- 
factor  of  96.5  per  cent. 

It  is  nearly  always  more  economical  to  supply  wattless  cur- 
rent in  a  power  station  from  unloaded  high  speed  synchronous 
motors,  than  from  the  main  synchronous  generators.  This  is 
more  especially  the  case  with  steam  turbine  or  very  slow-speed 
units,  as  such  "machines  cannot  be  economically  designed  with  the 
good  regulation  and  the  margin  on  the  field  magnets  necessary 
to  handle  satisfactorily  a  low  power-factor  load.  A  machine  of 
either  of  these  types  to  carry  satisfactorily  a  certain  kilowatt 
load  at  power-factor  70  per  cent  will  be  about  double  the  weight 
of  a  unit  to  carry  the  same  kilowatt  rating  at  unity  power- 
factor.  In  the  station  considered  in  the  preceding  paragraph,  we 
assumed  that  the  synchronous  generators  supplied  the  wattless 
current  required  for  the  induction  generators  and  outside  circuit. 
It  would  be  better,  however,  to  make  the  synchronous  generators 
1,000  K.V.A.  units  with  poor  regulation,  and  install  also  a  1,500 


THE   NON-SYNCHRONOUS  GENERATOR  95 

K.V.A.  high  speed  synchronous  motor  to  supply  all  the  wattless 
current.  This  would  give  a  cheaper  and  more  flexible  installa- 
tion, as  we  would  be  able  to  run  the  induction  generators  without 
the  synchronous  units  at  any  time,  using  the  synchronous  motor 
to  excite  them.  The  induction  units  would  require  no  direct 
current  exciters,  exciting  circuits,  or  switch  panels,  while  they 
would  probably  be  cheaper  than  the  synchronous  genera- 
tors, simpler  to  handle,  and  less  liable  to  break  down.  The 
above  is  sufficient  to  show  that  this  type  has  such  important 
advantages  that  it  should  be  carefully  considered  in  each 
individual  case  before  deciding  to  adopt  synchronous  generators 
exclusively. 

Other  Applications  of  Induction  Generators. — In  the  above 
remarks,  the  non-synchronous  generator  has  been  dealt  with 
more  especially  as  a  steam  turbine  driven  unit  for  generating 
alternating  current.  This  type,  however,  often  presents  im- 
portant advantages  for  other  and  more  especial  conditions;  two 
of  the  most  important  of  which  are  gas  engine  driven  alternators, 
and  steam  turbine  driven  direct  current  units.  The  advantage 
of  the  induction  generator  for  gas  engine  driven  units  is  that  it 
does  not  require  the  extreme  uniformity  of  speed  required  by  a 
synchronous  generator;  and  the  advantage  of  its  application  for 
direct  current  generation  by  turbine  units  is  that  by  the  use  of 
an  induction  generator  and  rotary  converter  we  can  avoid  the 
use  of  a  direct  current  turbo  generator. 

Gas  Engine  Driven  Units. — With  the  modern  tandem  and 
twin-tandem  gas  engines,  giving  respectively  two  and  four  im- 
pulses per  revolution,  gas  engine  driven  alternators  can  un- 
doubtedly be  run  in  parallel.  But  to  obtain  the  same  kind  of 
satisfactory  operation  that  is  obtained  with  steam  engines, 
large  fly-wheels  and  heavy  dampers  on  the  pole-faces  of  the 
alternators  are  necessary.  Such  fly-wheels  result  in  a  consider- 
able increase  in  cost,  and  sometimes  in  the  floor  space  taken  up 
by  the  engine;  and  also  a  loss  in  efficiency  due  to  the  increased 
bearing  friction  and  windage;  while  there  is  necessarily  a  con- 
siderable loss  in  the  dampers  on  the  pole-faces  of  such  a  gas 


96  THE   NON-SYNCHRONOUS  GENERATOR 

engine  driven  alternator,  because  the  irregularity  of  speed  in  a 
gas  engine  is  usually  such  that  the  dampers  have  to  perform 
heavy  work  in  accelerating  and  retarding  the  fly-wheels.  We 
can  readily  see  that  there  can  be  easily  an  increased  loss  of  three 
to  five  per  cent  from  these  two  causes,  which  would  not  be  de- 
tected except  in  a  gas  consumption  test,  when  running  in  parallel 
with  other  units.  Instead  of  synchronous  units,  we  can  install 
induction  generators,  and  have  high-speed  synchronous  motors 
running  light,  to  provide  the  necessary  lagging  current  for  the 
outside  circuit  and  for  exciting  the  generators.  In  this  case  any 
change  in  load  comes  first  on  the  synchronous  motors,  causing 
a  change  in  their  speed,  and  as  a  result  a  transference  of  the 
change  in  load  to  the  generator.  As  the  voltage  of  the  generator 
would  be  decided  by  the  excitation  of  the  synchronous  motors, 
the  voltage  regulation  of  the  station  is  that  of  the  motors;  so 
that  for  constant  potential  it  may  be  advisable  in  some  cases  to 
control  the  motor  excitation  by  an  automatic  voltage  regulator. 
The  size  of  the  direct  current  exciter  necessary  for  the  synchron- 
ous motors  would  depend  on  the  power-factor  of  the  load  on  the 
station,  and  would  be  greater,  the  greater  the  lagging  current 
required  by  the  external  circuit.  Generally  speaking,  the  size 
of  the  exciter  required  would  be  from  one-quarter  to  one-half 
of  that  necessary  for  the  corresponding  synchronous  generators. 
The  probable  arrangements  would  be  a  direct  connected  exciter 
on  each  synchronous  motor,  which  could  also  be  used  as  a  start- 
ing motor;  and  one  gas  engine  driven  exciter  for  starting  up  the 
first  motor. 

Taking  as  the  load  on  such  a  2,200  volt,  25  cycle  power 
station,  20,000  K.V.A.  at  70  per  cent  power-factor,  we  would 
have  four  5,000  K.V.A.,  75  R.P.M.  synchronous  generators,  each 
requiring  a  125  K.W.  exciter;  or  four  3,500  K.W.,  76  R.P.M. 
induction  generators,  together  with  four  4,500  K.V.A.,  500 
R.P.M.  synchronous  motors  with  60  K.W.  direct  coupled  starting 
motor  exciters.  Each  synchronous  motor  would  supply  the  1,000 
K.V.A.  exciting  current  required  by  one  induction  generator, 
together  with  3,500  K.V.A.  wattless  current  for  the  external 


THE  NON-SYNCHRONOUS  GENERATOR  97 

circuit.   The  relative  efficiencies  on  the  load  of  70  per  cent  power- 
factor  are  as  follows : 


Synchronous 
Generator. 

Induction  Generator 
and  Synchronous 
Motor. 

Full  load 

95  7 

94  0 

075    " 

95  1 

93  6 

050    " 

94  0 

92  5 

These  efficiencies  given  for  the  synchronous  generator  do  not 
take  into  account  the  additional  losses  due  to  the  increased 
friction  of  the  larger  fly-wheel,  and  the  losses  in  the  dampers  or 
solid  pole-faces  which  occur  with  the  gas  engine  driven  syn- 
chronous generator.  These  additional  losses  will  reduce  the 
efficiency  of  the  synchronous  generator  below  that  of  the  induc- 
tion generator  set.  The  cost  of  the  electrical  equipment  would 
not  be  very  different  in  the  two  cases,  and  as  the  larger  fly-wheel, 
shaft,  and  bearings,  required  for  the  synchronous  generator 
would  increase  the  cost  considerably,  it  is  probable  that  the 
induction  generator  equipment  would  be  somewhat  cheaper. 
It  must  be  remembered  that  this  is  an  extreme  case,  because  the 
low  power-factor  of  70  per  cent  taken  for  the  outside  load  is  very 
much  against  the  induction  generator,  and  that  if  the  power- 
factor  were  higher,  the  result  would  be  more  favorable  to  that 
type. 

It  might  be  supposed  that  unless  the  gas  engine  was  provided 
with  a  very  heavy  fly-wheel,  there  would  be  the  same  trouble 
with  hunting  of  the  induction  generator  and  synchronous  motor 
that  we  would  have  with  the  synchronous  generator.  But  such 
is  not  the  case.  There  will  undoubtedly  be  cross  currents  be- 
tween the  machines,  the  magnitude  of  which  will  depend  on  the 
variation  in  the  speed  of  the  gas  engine,  but  it  will  be  practically 
impossible  to  break  them  out  of  step.  The  worst  effect  of  this 
interchange  of  current  between  the  machines  is  the  heating  and 
losses  in  the  armature  conductors,  and  the  pulsation  in  voltage 
due  to  the  interchange  of  wattless  current.  This  pulsation  of 

7 


98  THE  NON-SYNCHRONOUS  GENERATOR 

voltage  is  diminished  by  dampers  on  the  pole-faces  of  the  syn- 
chronous motor,  but  it  will  usually  be  perceptible  when  only  one 
generator  is  running.  However,  as  these  gas  engine  driven  units 
will  be  used  mainly  for  power  work  in  mills,  the  slight  pulsation 
of  voltage  will  be  unimportant. 

It  is  by  no  means  settled  that  extremely  heavy  fly-wheels  and 
powerful  dampers  are  a  practical  and  advisable  solution  of  the 
parallel  running  difficulties  with  gas  engine  driven  synchronous 
generators.  So  though  induction  generators  are  not  put  forward 
as  the  only  solution  of  the  gas  engine  driven  alternator  question, 
they  probably  offer  the  most  practical  solution,  and  the  one  which 
will  recommend  itself  most  highly  to  the  conservative  power 
station  engineer  and  manufacturer. 

Steam  Turbine  Driven  Direct  Current  Units. — The  other 
special  application  of  the  non-synchronous  generator  above  re- 
ferred to — which  is,  to  operate  with  a  rotary  converter  to  supply 
direct  current — is  to  meet  the  special  case  in  which  a  steam 
turbine  is  desired  as  a  prime-mover  in  the  production  of  direct 
current  by  large  units.  Minimum  overhead  clearance,  small 
floor  space,  poor  foundations,  objection  to  the  vibration  of 
reciprocating  engines,  high  steam  economy  required  over  a  wide 
range  of  loads,  reduced  maintenance  and  supervision,  might 
render  the  adoption  of  a  steam  turbine  necessary;  and  as  the 
direct  current  turbo  generator  has  as  yet  hardly  established  its 
position  as  a  conservative  and  reliable  machine,  at  any  rate  in 
large  units,  it  would  be  necessary  to  use  some  type  of  alternating 
current  generator  in  combination  with  a  motor-generator  or 
rotary  converter.  Two  years  ago  the  Milwaukee  Electric  Rail- 
way and  Light  Co.  wished  to  install  an  auxiliary  3,000  K.W., 
300  volt  direct  current  steam-driven  generating  equipment  in 
the  basement  of  their  new  Public  Service  Building.  As  the  head- 
room was  limited  to  about  12  feet,  and  as  the  vibration  would  be 
objectionable,  reciprocating  engines  were  out  of  the  question. 
They  installed  two  1,500  K.W.  horizontal  steam  turbine  driven 
synchronous  alternators,  and  two  synchronous  motor-generator 
sets.  This  would  have  been  an  ideal  case  for  an  induction  gen- 


THE  NON-SYNCHRONOUS  GENERATOR 


99 


erator  and  rotary  converter.  The  rotaries  could  be  started  from 
the  direct  current  system,  and  when  up  to  speed  would  excite  the 
non-synchronous  generators;  no  exciters  nor  exciting  circuits 
would  be  necessary,  the  voltage  being  controlled  by  the  excita- 
tion of  the  rotaries.  The  equipment  which  was  installed  has  a 
combined  full-load  efficiency  of  about  88  per  cent,  while  the  com- 
bined efficiency  of  an  induction  generator  and  rotary  converter 
to  do  the  same  work  would  have  a  full-load  efficiency  of  about 
95  per  cent,  and  in  addition  it  would  probably  have  cost  about 
one-third  less. 

In  comparing  a  turbine  driven  non-synchronous  generator 
and  rotary  with  a  steam  engine  driven  direct  current  generator, 
the  former  is  found  to  be  a  more  flexible  equipment,  one  which 
will  carry  heavier  overloads;  and  is  usually  cheaper.  Any 
compounding  desired  can  be  obtained  by  means  of  a  compound 
winding  on  the  rotary;  while  by  use  of  transformers  we  can 
operate  rotaries  at  different  voltages  and  supply  different  direct 
current  systems.  The  rotary  can  be  located  at  a  distance  and 
the  power  transmitted  to  it  at  a  high  voltage,  or  the  equipment 
can  be  divided  up  and  arranged  as  we  please,  while  the  efficiency 
is  slightly  higher  on  the  induction  generator  equipment  than  on 
the  engine  type,  as  can  be  seen  from  the  following  table  of  full 
load  efficiencies  for  270-volt  generators. 


1000  K.W. 

2000  K.W. 

3000  K.W. 

Steam  turbine  driven  induction  generator  
Rotarv  converter  

97.5 
97.0 

98.0 
97.5 

98.25 
97.75 

Combined  efficiency  of  induction  generator 
and  rotary  converter  

94.5 

95.5 

96.0 

Engine  type  generator 

93  5 

94  25 

94  5 

There  are  many  indications  that  the  day  of  the  large  engine- 
type  direct-current  generators  is  past.  The  inaccessibility  of 
the  brushes,  the  difficulty  of  building  and  maintaining  a  commu- 
tator of  large  diameter,  and  the  numerous  other  drawbacks  of 
this  type  of  machine  have  caused  it  to  be  regarded  as  an  un- 


100  THE   NON-SYNCHRONOUS  GENERATOR 

desirable  addition  to  a  power  station.  It  is  a  question  whether 
the  steam  turbine  driven  induction  generator  and  rotary  con- 
verter are  not  superior  to  the  engine  driven  direct  current  gen- 
erator in  almost  every  case,  and  this  combination  should  always 
be  carefully  considered  when  any  new  direct-current  station  is 
laid  out,  or  any  extensions  are  added  to  existing  plants. 

I  have  endeavored  to  show  that  the  one  great  disadvantage 
of  the  induction  generator — its  inability  to  carry  a  lagging 
wattless  current  load — should  not  always  prevent  its  successful 
adoption;  and  that  the  important  advantages  it  possesses  in  its 
excellent  mechanical  construction,  high  efficiency,  good  charac- 
teristics in  regard  to  short-circuits  and  resonance,  strong  balanc- 
ing and  damping  action,  absence  of  rotating  windings  or  collector 
rings,  absence  of  direct-current  excitation  and  exciting  circuits, 
ease  of  parallel  running,  facility  for  control  of  load  by  governor, 
and  general  simplicity  and  flexibility  of  operation,  render  it  in 
many  cases  the  most  advisable  machine  to  adopt.  This  type  of 
generator  suffers  from  the  fact  that  it  was  judged  and  condemned 
in  the  early  days  of  electric  power  stations.  At  that  time  there 
was  no  real  field  for  this  generator;  but  the  introduction  of  the 
steam  turbine  and  the  gas  engine,  with  the  modern  development 
of  large  power  stations,  have  so  fundamentally  altered  condi- 
tions, that  the  induction  generator  is  no  longer  an  interesting 
curiosity,  but  one  of  the  most  promising  types  for  power  station 
equipment.  At  the  present  time  this  machine  can  be  considered 
as  having  been  placed  on  a  demonstrated  commercial  basis; 
and  while  it  may  have  limitations,  it  possesses  so  many  advan- 
tages, and  its  sphere  of  usefulness  is  so  extensive,  that  it  must 
be  acknowledged  to  offer  greater  future  possibilities  than  almost 
any  other  type  of  power  station  equipment. 

NOTE 

The  following  data  on  the  steam  consumption  obtained  from 
tests  at  the  Baltimore  Copper  Smelting  and  Rolling  Co.  plant 
referred  to  earlier  in  the  paper,  are  of  interest  in  comparing  the 


THE   NON-SYNCHRONOUS 


efficiencies  of  the  two  types  of  direct  current  generating  equip- 
ment. The  first  column  gives  the  actual  steam  consumption  of 
the  1,200  K.W.  steam  turbine  driven  induction  generator  and 
rotary  converter,  the  turbine  being  run  with  140  Ib.  steam  pres- 
sure, 28  in.  vacuum  and  135°  superheat.  The  second  column 
gives  the  corresponding  figures  on  a  Corliss  engine  and  direct 
connected  generator,  the  steam  consumption  being  based  on  a 
minimum  consumption  of  12.5  Ib.  per  I.H.P.  at  80  per  cent  of  full 
load. 

POUNDS  OF  STEAM   PER   KILOWATT  HOUR   AT  DIRECT 
CURRENT  TERMINALS 


Load. 

Steam  Turbine 
Equipment. 

Corliss  Engine 
Equipment. 

0.5  load 
0.75    " 
1. 
1.25    " 

21.2  Ib. 

18.2 
17.5 
17.5 

23.5  Ib. 
19.8 
20.3 
23.2 

[Presented  before  the  American  Institute  of 
Electrical  Engineers,  June  29,  1908.] 


MODERN    DEVELOPMENT   IN   SINGLE   PHASE 
GENERATORS 


THE  single  phase  alternator  has  been  in  commercial  use  now 
for  twenty  years,  and  it  may  seem  surprising  that  there  should 
be  new  developments  at  this  late  date.  However,  single  phase 
alternators  have  been  used  in  the  past  almost  exclusively  for 
lighting  work,  and  in  units  of  comparatively  small  output  and 
low  speed.  Recently,  on  account  of  the  adoption  of  single  phase 
current  for  traction  work,  an  important  demand  has  arisen  for 
large,  high  speed,  low  frequency,  single  phase  generators;  and  it 
is  in  the  design  and  manufacture  of  such  units  that  the  engineer 
has  had  to  overcome  new  difficulties.  In  these  large,  high  speed 
machines  for  15  and  25  cycles  the  difficulties  met  with,  are  due 
almost  entirely  to  the  large  pole-pitch  and  high  armature  reaction 
which  it  is  necessary  to  adopt.  A  500  K.W.,  60  eye  e,  72  pole, 
single  phase  generator  would  have  a  pole-pitch  of  about  7  in.; 
while  a  6,000  K.W.,  15  cycle,  2  pole  machine  would  have  one 
one  of  about  120  in.  It  is  easily  seen  that  the  design  of  these 
two  generators  will  be  radically  different. 

These  difficulties  in  single  phase  generators  of  large  pole- 
pitch  are  the  result  of: 

1.  Pulsation  of  the  armature  reaction. 

2.  Mechanical  stresses  on  the  ends  of  the  armature  coils, 
where  they  are  not  embedded  in  the  laminated  core. 

The  pulsation  of  the  armature  reaction  causes  hysteresis  and 
eddy-current  losses  throughout  the  machine,  often  resulting  in 

103 


104  SINGLE   PHASE   GENERATORS 

dangerous  heating  and  low  efficiency.  The  mechanical  stresses 
on  the  ends  of  the  armature  coils,  due  to  the  current,  result 
in  vibration  or  distortion  of  the  windings,  and  often  in  damage 
to  the  insulation  or  complete  destruction  of  the  coils;  these 
stresses  being  particularly  serious  in  single  phase  railway  genera- 
tors, on  account  of  the  sudden  variations  in  load  and  frequent 
short-circuits  to  which  these  machines  are  subjected.  As  the 
effect  of  the  mechanical  stresses  on  the  armature  coils,  and  the 
losses  due  to  the  pulsation  of  armature  reaction,  increase  approxi- 
mately proportionally  to  the  square  of  the  pole-pitch  in  gen- 
erators of  standard  design,  it  is  easily  seen  why  such  effects 
which  were  negligible  in  the  old  single  phase  alternators  of  small 
pole  pitch  have  become  quite  important  in  the  modern  steam 
turbine  driven  generator.  The  seriousness  of  these  difficulties 
when  first  met  with  was  so  great,  that  even  within  the  last  two 
years  responsible  engineers  have  stated  it  was  impossible  to 
build  satisfactory  low  frequency,  high-speed,  single  phase  gen- 
erators of  large  capacity;  and  it  is  only  by  careful  study  and 
experimenting  that  the  modern  machine  of  this  type  has  been 
developed. 

Losses  Due  to  Pulsation  of  Armature  Reaction. — In  a  poly- 
phase generator  the  armature  current  produces  a  magnetic  flux 
which  rotates  synchronously  with  the  field  magnet.  This  mag- 
netic flux  may  result  in  increased  losses  in  the  laminated  arma- 
ture core,  but  being  of  practically  constant  magnitude,  causes 
very  little  loss  in  the  iron  of  the  magnetic  circuit.  On  the  other 
hand,  the  armature  current  in  a  single  phase  generator  produces 
a  pulsating  magnetic  flux  which  is  stationary  in  space,  and  it  is 
easily  seen  that  this  flux  will  cause  hysteresis  and  eddy-current 
losses  throughout  the  whole  magnetic  circuit.  The  exact  effect 
of  the  armature  reaction  flux  on  the  rotating  magnets  depends, 
of  course,  on  the  relative  phase  of  the  armature  current  and 
electromotive  force;  that  is,  on  the  power-factor  of  the  load 
carried  by  the  generator.  When  the  power-factor  is  unity  and 
the  armature  current  is  in  phase  with  the  electromotive  force, 
the  armature  reaction  flux  is  a  cross-magnetization;  when  the 


SINGLE  PHASE   GENERATORS  W5 

power-factor  is  zero  and  the  armature  current  is  90°  out  of  phase 
with  the  electromotive  force,  the  armature  reaction  flux  is  a 
demagnetization.  In  the  special  case  in  which  the  rotating  field 
magnet  is  cylindrical,  without  projecting  poles,  the  effect  of  the 
armature  reaction  flux  on  the  magnets  is  more  nearly  independ- 
ent of  the  power-factor  of  the  armature  current.  But  in  any 
case  this  flux  is  a  pulsating  one,  and  there  are  important  losses 
in  the  field  magnets,  due  to  their  rotation  through  this  cross 
magnetizing  or  demagnetizing  flux.  An  estimate  of  the  com- 
bined losses  in  the  armature  and  field  magnets  due  to  the  pulsat- 
ing armature  reaction,  can  be  obtained  in  a  number  of  ways. 

We  can  measure  the  increase  of  the  power  required  to  rotate 
the  field  magnets  due  to  normal  R.M.S.  current  in  the  armature 
coils,  with: 

1.  Direct  current  in  the  armature  winding. 

2.  Alternating  current  of  synchronous  frequency  in  the  arma- 
ture. 

3.  Armature  short-circuited  and  field  excited. 
Or  with  the  magnets  stationary  we  can : 

4.  Send  normal  frequency  alternating  current  through  the 
armature  and  measure  the  losses  by  a  wattmeter. 

These  methods  must  all  be  regarded  as  convenient  tests 
which  are  found  by  experience  to  give  some  indication  of  the 
magnitude  of  the  losses.  Method  4  has  the  additional  advan- 
tage that  we  can  vary  the  relative  position  of  the  armature  re- 
action flux  and  the  pole-faces,  and  thus  investigate  the  variation 
of  the  losses  in  a  single  phase  generator  with  the  power-factor  of 
the  load. 

The  only  exact  methods  of  measuring  the  losses  are: 

1.  As  unknown  losses  in   a   motor-generator    (Hopkinson) 
method  efficiency  test. 

2.  From  a  comparison  of  the  temperature  rises  obtained  on 
full  load  with  those  obtained  with  known  losses. 

Unfortunately,  both  of  these  tests  are  difficult  to  make 
accurately,  especially  on  a  large  machine,  and  probably  in 
practice  they  do  not  give  results  which  are  any  more  accurate 


106  SINGLE  PHASE   GENERATORS 

than  the  other  more  approximate  methods.  So  at  the  present 
time  we  have  to  acknowledge  that  though  we  know  a  great  deal 
about  the  relative  values  of  the  losses  under  various  conditions, 
our  knowledge  of  their  absolute  values  is  limited. 

Pole-Face  Dampers. — Losses  caused  by  a  pulsating  flux  in  the 
magnetic  circuit  are  due  to: 

1.  Hysteresis. 

2.  Eddy  currents. 

And  the  relative  magnitudes  of  the  two  depend  on  the  amount 
of  solid  metal  in  the  path  of  the  flux.  If  the  whole  magnetic 
circuit  is  laminated,  then  the  losses  are  practically  all  due  to 
hysteresis.  On  the  other  hand,  if  we  have  solid  cast  steel  poles 
there  will  be  eddy  currents  in  these  poles  which  will  partly 
choke  back  the  pulsation  of  the  flux  so  that  the  hysteresis  loss  will 
be  reduced.  But  in  this  case  there  will  be  eddy-current  losses 
in  addition  to  the  hysteresis,  and  the  extent  to  which  the  total 
loss  is  changed  will  depend  on  the  proportions  and  design  of  the 
magnetic  circuit.  If  we  place  a  heavy  copper  damper  in  the 
path  of  the  pulsating  flux,  it  will  provide  a  low-resistance  path 
for  the  eddy  currents,  so  that  the  pulsating  flux  and  consequent 
hysteresis  loss  will  be  reduced  practically  to  zero,  while  on 
account  of  the  low  resistance  of  the  damper  circuit  the  eddy 
loss  will  not  be  appreciable.  The  extent  to  which  the  losses  and 
the  pulsation  of  the  flux  vary  according  to  the  presence  of  eddy 
currents,  can  be  determined  for  any  particular  design,  by  varying 
the  thickness  of  the  lamination,  or  by  changing  to  solid  poles  or 
the  addition  of  dampers.  It  is  usually  found  that  the  losses  are 
greatest  with  heavy  laminations  or  solid  poles;  that  they  are  less 
for  thin  laminations,  and  practically  zero  when  heavy  low- 
resistance  dampers  are  used  either  with  solid  or  laminated  poles. 
Fig.  25  shows  the  pulsation  of  the  armature  reaction  flux  in 
a  500  K.W.,  single  phase,  20-pole  generator,  as  determined  by 
means  of  search-coils  wound  on  the  pole-faces.  C  shows  the 
pulsation  for  laminated  poles,  No.  29  gauge;  B  shows  the  same 
machine  with  solid  poles;  and  A  the  same  solid  pole-faces  covered 
with  a  f-in.  copper  plate.  The  magnitude  of  the  pulsations  in 


SINGLE  PHASE   GENERATORS  107 

the  three  cases  is  about  in  the  ratio  of  from  30  to  15  to  1 ;  thus 
the  copper  plate  has  practically  damped  out  all  pulsations,  the 
armature  reaction  flux  becoming  constant.  In  practice,  a  copper 
damper  usually  takes  a  form  similar  to  the  squirrel-cage  second- 
ary of  an  induction  motor.  Heavy  copper  bars  are  dovetailed 
into  the  pole-faces,  and  short-circuited  at  the  ends  by  copper 
rings  or  discs.  Fig.  26  shows  such  a  cage  damper  on  the  field 
magnet  of  a  6,000  K.W.,  2  pole  generator. 

The  question  of  losses  due  to  the  pulsating  armature  reaction 
in  a  single  phase  generator  may  be  considered  in  another  and 


V 


Time 
FIG.  25. 

possibly  a  simpler  way.  The  single  phase  pulsating  field  is 
equivalent  to,  and  produces  the  same  effect  as,  two  rotating 
fields  each  of  half  its  maximum  value,  one  rotating  at  the  same 
speed  and  in  the  same  direction  as  the  generator  field  magnet, 
and  the  other  rotating  at  the  same  speed  but  in  the  opposite 
direction.  The  flux  rotating  synchronously  with  the  field 
magnet,  being  constant  in  magnitude,  causes  very  little  loss. 
The  flux  rotating  in  the  reverse  direction  causes  losses  through- 
out the  whole  magnetic  circuit  due  to  hysteresis  and  eddies.  If 
a  squirrel-cage  damper  encloses  the  field  magnets,  this  damper 


108 


SINGLE  PHASE   GENERATORS 


system  acts  in  regard  to  this  reverse  rotating  flux  in  the  same 
way  as  the  short-circuited  secondary  of  an  induction  motor  or 
transformer;  a  current  is  induced  in  the  damper  which  produces 
a  field  neutralizing  the  rotating  flux.  The  eddy  and  hysteresis 
loss  in  the  iron  of  the  magnetic  circuit  which  would  be  caused 
by  this  rotating  flux  is  thus  eliminated,  and  the  only  loss  is  that 
due  to  the  current  circulating  in  the  damper.  If  we  make  the 
conductors  forming  the  squirrel  cage  of  sufficiently  low  resistance, 
this  damper  loss  becomes  negligible,  with  the  result  that  the 
entire  loss  due  to  the  pulsating  armature  reaction  of  the  single 
phase  generator  is  practically  eliminated. 

To  show  how  serious  this  matter  of  losses  becomes  in  high 
speed,  two  pole,  single  phase  machines  without  dampers,  the 
following  table  is  given,  showing  the  loss  and  full  load  tempera- 
ture rise  on  three  turbo  generators  at  full  load,  both  with  and 
without  dampers: 

TWO  POLE,  25  CYCLE,  SINGLE  PHASE  GENERATORS.  SAME 
CURRENT  PER  ARMATURE  CONDUCTOR  ONE  AND  THREE- 
PHASE,  UNDER  ALL  CONDITIONS,  AND  ALL  LOSSES  IN  PER 
CENT  OF  SINGLE  PHASE  RATING 


THREE  PHASE. 

SINGLE  PHASE. 

Generator 

TTiolH 

Without  Dampers. 

Without  Dampers. 

With  Dampers. 

Rating. 

Loss 
Per 

Cent. 

Temp. 
Rise, 
Degrees 
Cent. 

Loss 
Per 
Cent. 

Temp. 
Rise, 
Degrees 
Cent. 

Loss 
Per 

Cent. 

Temp. 
Rise, 
Degrees 
Cent. 

750  K.W. 

Solid 

0.53 

27 

3.75 

95 

0.8 

34 

1000     " 

Solid 

0.3 

31 

3.0 

122 

0.5 

37 

1000     " 

Laminated 

0.2 

19 

3.8 

150 

0.3 

18 

It  will  be  seen  that  in  these  three  machines,  operating  single 
phase,  there  is  due  to  the  pulsating  flux  an  average  loss  of  3.5  per 
cent  and  an  average  temperature  rise  of  125°  C.,  without 
dampers;  with  dampers  the  average  loss  is  0.5  per  cent  and  the 
temperature  rise  30°  C.  Figures  are  given  only  on  compara- 
tively small  machines  on  account  of  the  difficulty  of  measuring 


SINGLE   PHASE   GENERATORS  109 

losses  on  largo  machines.  But  tests  on  larger  generators  up  to 
6,000  K.W.  capacity  show  that  the  improvement  due  to  heavy 
copper  dampers  is  even  more  striking  in  large  machines  than  it 
is  in  small.  So  far  as  experience  goes  at  the  present  time,  it 
may  he  said  that  the  use  of  such  dampers  is  the  complete  solution 
of  the  difficulties  due  to  pulsating  armature  reaction  met  with 
in  large,  low  frequency,  two  pole,  single  phase  generators. 

Mechanical  Stresses  on  Armature  Coils. — That  it  was  necessary 
to  mechanically  brace  the  end  connections  of  the  armature 
coils  on  a  direct  current  machine  subjected  to  sudden  loads  and 
short-circuits  has  been  known  for  many  years.  But  until 

f-  --rr^  -•* 


FIG.  26.— Rotary  Field  Magnet  for  6,000   K.W.,   2   Pole,   25  Cycle,   Single 

Phase  Generator. 

quite  recently  additional  supports  for  alternator  armature  coils 
were  seldom  provided.  The  reason  for  this  was  that  as  the  con- 
tinuous short-circuit  current  of  an  alternator  is  only  about  two 
or  three  times  normal,  it  was  not  considered  that  the  mechanical 
stresses  on  the  ends  of  the  small  pole  pitch  coils  generally  in 
use  were  sufficiently  great  to  cause  trouble.  Only  during  the 
last  few  years  has  it  been  demonstrated  by  experience  that  coil 
supports  on  large  pole  pitch  alternators  are  not  only  advisable, 
but  necessary,  and  that  on  account  of  the  numerous  short-circuits 


110  SINGLE   THASE   GENERATORS 

they  are  particularly  necessary  on  single  phase  machines  operat- 
ing on  traction  circuits. 

When  an  alternator  is  suddenly  short-circuited,  the  first  rush 
of  current  is  limited  only  by  the  self-induction  and  resistance  in 
circuit;  and  in  the  case  of  an  alternator  of  low  self-induction, 
this  first  rush  of  current  on  sudden  short-circuit  may  be  20  to  30 
times  normal  full  load  current.  As  the  mechanical  stress  on  the 
end  of  the  armature  coils  varies  with  the  square  of  the  current, 
the  stress  on  the  armature  coils  will  be  400  to  900  times  normal. 
A  6,000  K.W.,  2  pole,  25  cycle,  single  phase  generator  has  a 
pole  pitch  of  about  100  in.,  and  the  length  of  the  end-connection 
at  one  end  of  one  armature  coil  will  be  about  180  in.  The 
mechanical  stress  on  the  end  connections  at  one  end  of  one  arma- 
ture coil  of  this  machine  on  a  sudden  short-circuit  is  approxi- 
mately 5  tons;  and  usually  on  low-frequency  high  speed  genera- 
tors of  large,  capacity,  the  mechanical  stresses  on  the  end 
connections  at  one  end  of  one  armature  coil  in  the  case  of  a  sud- 
den short-circuit  are  from  2  to  10  tons.  When  it  is  considered 
that  this  results  in  a  sudden  mechanical  shock  to  the  winding,  we 
can  realize  the  strength  of  the  coil  supports  required,  and  can 
understand  the  disastrous  results  sometimes  obtained  on  short- 
circuits,  when  such  supports  are  omitted.  It  is  obvious  from  these 
stresses  that  coil  supports  must  be  of  metal  and  of  heavy  cross- 
section.  The  difficulty  of  suitably  insulating  metal  coil  sup- 
ports has  caused  numerous  other  materials  to  be  used,  but 
though  supports  of  wood,  porcelain,  and  similar  insulating 
materials  have  been  tried,  it  is  easily  understood  that  they  have 
proved  unsatisfactory  on  machines  of  large  pole  pitch.  Fig. 
27  shows  a  form  of  coil  support  and  bracing  which  has 
proved  very  satisfactory  for  such  machines.  It  consists  of  a 
bronze  strap  which  clamps  the  coils,  by  means  of  wood  insulating 
blocks  and  insulated  bronze  bolts,  to  malleable  iron  brackets, 
which  also  serve  to  support  the  copper  strap  connectors  between 
the  various  groups  of  coils.  The  support  and  its  method  of  ap- 
plication are  evident  from  the  illustration;  it  is  placed  in 
position  after  the  machine  is  wound  and  is  removable  in  a  few 


SINGLE   PHASE   GENERATORS 


111 


FIG.  27.— Armature  Winding  of  10,000  K.W.,  11,000  Volt,  60  Cycle,  4  Pole 
Generator  Showing  Method  of  Bracing  Coils. 


112  SINGLE   PHASE   GENERATORS 

minutes  at  any  time.  It  is  not  suggested  that  this  is  the  only 
satisfactory  type  of  coil  support  that  can  be  used;  it  is  merely 
given  as  a  type  which  has  proved  successful  in  actual  operation 
on  machines  up  to  10,000  K.W.  capacity,  and  which  has  appar- 
ently solved  the  difficulties  due  to  mechanical  stresses  on  the 
end  connecions  of  large  pole  pitch  generators  liable  to  sudden 
variations  in  load  or  frequent  short-circuits. 

The  two  main  difficulties  met  with  in  large  low  frequency, 
high  speed,  single  phase  generators,  that  have  been  described 
above,  can  at  the  present  time  be  regarded  as  having  been  suc- 
cessfully overcome.  The  use  of  heavy  copper  dampers  on  the 
pole-faces,  and  heavy  bronze  coil  supports  applied  to  the  ends 
of  the  armature  coils,  in  such  a  way  as  to  take  directly  the 
mechanical  stresses  which  develop  on  short-circuits,  has  now 
made  such  generators  a  practical  success.  Like  every  other 
new  type  of  electrical  machinery,  the  large  turbine-driven, 
single  phase  generator  has  had  its  period  of  development,  but 
at  the  present  time  it  may  be  said  that  such  generators  for  15 
and  25  cycle,  in  units  of  5,000  to  10,000  K.W.  capacity,  can  be, 
and  are,  built  with  the -same  success  as  that  obtained  on  slow 
speed  polyphase  generators. 


[Presented  before  the  National  Electric  Light 
Association,  June  3,  1909.] 


PERFORMANCE   SPECIFICATIONS   AND   KATINdS 


[' itiforiiiitij  <>f  /{aliny. — The  question  of  uniformity  in  the 
rating  of  generators  and  motors  is  in  a  decidedly  unsatisfactory 
condition  at  the  present  time,  a<  almost  every  manufacturer  and 
every  purchaser  rates  his  generators  or  motors,  and  specifies  their 
performance  in  a  different  way.  The  result  of  this  is  that  when 
an  operator  decides  a  new  generator  is  required  he  specifies  as 
nearly  as  possible  a  unit  he  considers  suitable  for  the  work,  buys 
one  that  the  manufacturer  estimates  will  fill  his  specifications, 
and  then  proceeds  to  test,  to  find  what  output  he  can  obtain  from 
it  under  the  conditions  that  exist  in  his  station.  The  operating 
engineer  and  the  manufacturer  are  both  to  blame  for  this  state 
of  affairs;  the  manufacturer  because  he  often  designs  and  builds 
his  machine  to  take  care  of  some  arbitrary  theoretical,  rather 
than  practical,  conditions  of  operation;  and  the  operating  en- 
gineer because  he  does  not  more  closely  study  the  operating 
conditions  of  his  machinery,  and  insist  that  the  manufacturer 
supply  generators  or  motors  suitable  to  his  requirements.  So 
long  as  the  mechanical  construction  of  a  machine  is  satisfactory 
and  such  that  repairs  can  be  easily  carried  out,  no  further  in- 
quiries are  usually  made  by  the  purchaser,  provided  the  name- 
plate  carries  the  nominal  rating  required.  The  tendency  is  to 
pay  altogether  too  much  attention  to  the  figures  stamped  on 
the  name-plate  when  buying  a  generator  or  motor.  The  result 
is  that  almost  every  engineer  knows  examples  of  units *with  the 
same  nominal  rating,  built  either  by  the  same  or  by  different 

8  113 


114  SPECIFICATIONS  AND   RATINGS 

manufacturers,  some  of  which  under  certain  operating  conditions 
are  capable  of  carrying  50  per  cent  more  load  than  others.  The 
remedy  for  this,  of  course,  is  to  have  some  uniform  and  rational 
method  of  rating  adopted  by  all  manufacturers,  and  then  to 
specify  and  select  machines  suitable  for  the  work  they  have  to 
perform. 

Temperature  and  Power-Factor. — The  two  most  important 
points  in  which  specification  and  performance  guarantees  are 
unsatisfactory  seem  to  be: 

1.  The  temperature  rise  on  alternating  and  direct  current 
generators  or  motors;  and 

2.  The  power-factor  of  the  load  for  which  guarantees  are 
made  on  alternating  current  generators. 

The  present  system  of  temperature  guarantees,  which  con- 
sists in  stating  the  maximum  temperature  rise  in  any  part  of 
the  machine,  under  numerous  different  conditions  of  load  and 
for  varying  periods,  is  both  unsatisfactory  and  irrational.  The 
temperature  limit  of  output  in  any  generator  or  motor  is  not 
decided  by  the  temperature  rise,  but  by  the  absolute  temperature 
of  the  insulation.  At  one  of  the  specified  loads  the  armature 
iron  may  be  the  hottest  part,  at  another  the  field  coils,  while  at  a 
third  it  may  be  the  armature  winding  or  commutator.  As  each 
of  these  parts  has  a  different  limiting  temperature  to  which  it 
can  be  subjected  without  damage,  it  is  obvious  that  such  a  sys- 
tem of  uniform  guarantees  is  misleading.  The  operating  engin- 
eer is  not  practically  interested  in  knowing  that  the  temperature 
rise  of  his  generator,  as  measured  by  a  thermometer  on  the  out- 
side of  the  winding,  is  35  degrees  on  full  load  for  24  hours,  45 
degrees  on  25  per  cent  overload  for  24  hours,  and  55  degrees  on 
50  per  cent  overload  for  one  hour.  What  he  must  know  is 
the  maximum  load  he  can  safely  carry  continuously,  and  in  some 
cases  the  overload  he  can  safely  carry  for  two  or  three  hours. 
This  load  depends,  obviously,  on  the  room  temperature  and  on 
the  limiting  temperature  to  which  the  insulation  can  be  sub- 
jected without  damage.  The  most  rational  method  of  tempera^ 
ture  rating  is,  then,  to  specify  the  maximum  continuous  rating 


SPECIFICATIONS  AND   RATINGS  115 

at  which  the  unit  can  be  safely  operated  with  a  certain  room 
temperature,  e.g.,  25  degrees  centigrade;  and  where  desirable  the 
safe  two  or  three  hour  overload,  with  the  same  room  tempera- 
ture, can  also  be  given.  Usually,  in  a  modern,  well  ventilated 
generator  or  motor,  the  temperature  reaches  its  maximum  after 
a  three  or  four  hour  run,  so  that  the  two  or  three  hour  overload 
is  about  the  same  as  the  maximum  continuous  rating.  In  this 
case  the  system  of  temperature  guarantees  reduces  to  a  single 
guarantee  of  the  maximum  safe  load  which  the  unit  can  carry 
continuously,  with  the  specified  room  temperature;  and  it  should 
be  noted  that  this  maximum  load  is  greater  the  lower  the  temper- 
ature of  the  air  cooling  the  machine.  This  system  of  maximum 
rating  has  been  in  use  to  some  extent  for  the  past  year,  and  it 
would  seem  that  the  sooner  it  is  adopted  universally,  the  sooner 
will  purchasers  have  a  rational  idea  in  regard  to  the  temperature 
limitations  of  the  machines  they  arc  buying. 

Few  engineers  seem  to  appreciate  the  effect  of  a  low  power- 
factor  on  the  operation  of  an  alternator,  and  few  operating  engi- 
neers consider  the  power-factor  of  the  load  an  important  point 
when  installing  new  machinery.  When  new  generators  are  to 
be  bought,  we  regularly  find  100  per  cent  power-factor  machines 
specified  for  a  station  operating  with  a  power-factor  varying  from 
65  to  85  per  cent;  and  the  purchaser  becomes  suspicious  when  he 
is  told  that  a  standard  100  per  cent  power-factor  generator  would 
not  carry  much  more  than  50  per  cent  of  its  rated  kilovolt- 
amperes  if  operating  on  his  system,  or  that  he  ought  to  buy  a 
more  expensive  generator  which  is  designed  and  guaranteed  for 
a  75  to  80  per  cent  power-factor  load.  A  standard  alternator 
designed  for  100  per  cent  power-factor  load  (i.e.,  for  rotary 
converter  or  synchronous  motor  work)  is  usually  designed  with 
a  comparatively  saturated  magnetic  circuit,  and,  unless  extreme- 
ly liberally  designed,  such  a  machine  will  not  hold  up  voltage  with 
full  rated  K.V.A.  at  80  or  90  per  cent  power-factor.  If  such  a 
machine  were  required  for  80  per  cent  power-factor,  it  would  be 
re-designed  with  an  unsaturated  magnetic  circuit,  given  a  rating 
of  about  75  per  cent  of  its  nominal  K.V.A.  rating  at  100  per  cent 


116  SPECIFICATIONS   AND   RATINGS 

power-factor,  and  possibly  a  higher  temperature  rise  specified  for 
the  field  coils. 

Any  method  of  giving  alternators  a  different  rating  for 
every  operating  power-factor  would  probably  be  too  complicated 
for  practical  work.  It  has,  therefore,  been  proposed  to  continue 
to  give  all  machines  a  nominal  rating  in  kilo  volt-amperes  at  100 
per  cent  power-factor,  and  in  addition  to  give  the  maximum  load 
which  they  will  safely  carry  at  various  lower  operating  power- 
factors.  This  maximum  load  at  low  power-factors,  is  decided  for 
some  machines  by  the  question  of  holding  up  voltage,  and  for 
others  by  the  heating  of  the  field  coils,  so  that  for  cases  in 
which  temperature  is  the  limit  the  maximum  load  should 
be  referred  to  a  definite  room  temperature,  e.g.,  25  degrees 
centigrade.  This  method  of  rating  alternators  gives  for  the 
purpose  of  comparison  a  nominal  rating  at  100  per  cent  power- 
factor,  and  in  addition  gives  the  purchaser  exact  information  as. 
to  the  operative  characteristics  of  the  proposed  unit  under  any 
particular  condition  of  load.  If  the  operating  engineer  knows 
the  power-factor  at  which  the  machine  will  be  required  to  operate, 
he  should  have  no  difficulty  in  deciding  as  to  the  suitability  of 
the  unit  for  his  requirements. 

The  question  of  power-factor  is  equally  important  in  rotary 
converters  and  synchronous  motors.  Generally  speaking,  the 
power-factor  should  always  be  adjusted  to  100  per  cent,  unless  for 
some  special  reason,  definitely  specified  and  understood  at  the 
time  the  machine  was  purchased.  A  case  recently  came  to  the 
writer's  knowledge  in  which  a  system  was  operating  its  rotaries 
at  90  per  cent  power-factor,  and  when  the  manufacturer  pro- 
tested on  behalf  of  his  generators  and  rotaries,  the  operating 
engineer  stated  that  he  considered  90  per  cent  a  "  mighty  good  " 
power-factor.  Possibly  it  would  be  for  induction  motors,  but  for 
rotary  converters  it  is  a  "  mighty  bad  "  one.  Synchronous  motors 
are  often  used  to  correct  the  power-factor  of  the  line,  but  when 
they  are  installed  with  that  intention  the  maximum  kiloyolt- 
amperes  at  the  required  power-factor  should  be  specified,  exactly 
as  in  the  case  of  an  alternating  current  generator.  A  synchron- 


SPECIFICATIONS  AND  RATINGS  117 

ous  motor  designed  to  operate  at  100  per  cent  power-factor,  is 
just  as  unsuitable  for  operating  at  a  low  power-factor  as  an 
alternator  would  be  in  a  similar  case. 

Limiting  Conditions. — Neglecting  the  question  of  efficiency, 
the  limit  for  operating  conditions  should,  in  all  cases,  be  decided 
by  the  resultant  injury  to  the  machine.  The  limit  of  tempera- 
ture rise  is  decided  by  the  damage  to  the  insulation,  or  to  the 
mechanical  construction,  of  the  part  of  the  machine  considered. 
Some  insulation  will  not  stand  continuously  a  temperature 
higher  than  90  degrees  centigrade  without  deterioration;  other 
insulation  will  stand  300  degrees  safely.  The  temperature  limit 
on  a  commutator  or  collector-rings  is  usually  decided  by  the 
shrinkage  of  insulation  or  unequal  expansion  of  the  materials, 
causing  loss  of  mechanical  balance,  or  loss  of  accuracy  on  the 
wearing  surface.  The  limit  of  the  allowable  sparking  on  a 
commutator  or  collector-rings  is  the  resulting  temperature  rise, 
the  damage  to  the  surface  of  the  commutator  or  collector-rings, 
and  the  disintegration  of  the  brushes.  All  these  effects  must  be 
considered  in  relation  to  the  duration  of  the  specified  load ;  and 
as  in  such  cases  it  is  difficult  for  the  purchaser  to  decide,  without 
an  actual  test,  the  amount  of  damage  that  will  result  from  a 
certain  condition  of  operation,  he  must,  to  a  great  extent,  de- 
pend upon  the  guarantees  of  the  manufacturer;  which  guaran- 
tees, however,  will  be  subsequently  checked  by  the  actual  opera- 
tion of  the  machine  in  service. 

Testing  and  Specifications. — Testing  to  determine  in  what 
degree  a  unit  meets  the  specified  detailed  performance  guarantees 
is  always  a  very  difficult  question.  It  is  almost  impossible  to 
get  an  accurate  direct  test  of  the  voltage  regulation  of  any 
alternator,  as  the  result  is  measured  only  as  the  difference  of 
two  high  readings,  and  a  variation  in  any  one  of  the  conditions 
of  test  affects  the  result.  Unless  made  in  a  laboratory,  away 
from  masses  of  iron  which  would  affect  the  accuracy  of  the  in- 
struments, an  input-output  efficiency  test  of  a  motor-generator 
can  not  be  made  with  a  greater  accuracy  than  2  or  3  per  cent  on 
account  of  the  impossibility  of  obtaining  accurate  instrument 


118  SPECIFICATIONS  AND  RATINGS 

readings  under  practical  conditions.  In  both  of  these  cases,  the 
direct  method  of  test  has  to  be  abandoned  in  favor  of  an  indirect 
method,  which  enables  more  accurate  results  to  be  obtained. 
Temperature  tests  are  very  difficult  to  carry  out  accurately,  and 
unless  careful  precautions  are  taken  by  experienced  observers, 
it  is  often  impossible  to  be  sure  of  results  to  5  degrees.  Generally 
speaking,  the  customer  will  more  profitably  spend  his  time  in 
investigating  the  conditions  of  operation,  and  in  specifying  a 
machine  suitable  to  operate  under  these  conditions,  than  in 
making  tests  to  determine  regulation,  temperature  rise,  and  other 
similar  characteristics  which  will  be  of  only  doubtful  accuracy  and 
value  when  made.  If  a  generator  or  motor  is  specified  suitable 
for  the  work,  then  the  most  satisfactory  and  convincing  test  for 
the  machine  is  the  manner  in  which  it  performs  its  work;  and  by 
means  of  suitable  performance  specification  this  should  be  made 
something  definite,  and  something  to  which  the  customer  can 
hold  the  manufacturer.  If  this  were  done,  we  should  have 
fewer  disputes  on  the  question  of  whether  or  not  a  machine 
satisfies  its  contract  guarantees,  we  should  have  fewer  unsuitable 
units  installed,  and  I  think  we  should  have  better  operating  con- 
ditions on  most  power  systems. 


[Presented  before  the  National  Electric  Light 
Association,  June  3,  1909.] 


INPUT-OUTPUT    EFFICIENCY    TESTS    ON    ROTARY 
CONVERTERS    AND    MOTOR-GENERATORS 


THE  objection  to  the  input-output  method  of  testing  the 
efficiency  of  rotary  converters  or  motor-generator  sets  is  in  its 
inaccuracy.  This  is  due  to  the  fact  that  an  error  of  one  per  cent 
in  any  reading  results  in  a  one  per  cent  error  in  the  efficiency; 
and  when  it  is  considered  that  it  is  very  difficult  to  duplicate  in- 
strument readings  with  a  greater  accuracy  than  from  one  or  two 
per  cent,  the  importance  of  this  objection  can  be  realized.  In 
such  a  test  we  have  two  readings  to  take,  one  of  the  input  and 
one  of  the  output ;  and  an  error  of  one  per  cent  in  each  of  these 
readings  may  result  in  an  error  of  two  per  cent  in  the  efficiency. 
The  reason  for  the  popularity  of  the  input-output  method  of 
measuring  efficiency  is  the  extreme  simplicity  of  the  test :  as  it 
only  requires  simultaneous  readings  to  be  taken  of  the  current, 
voltage,  and  watts  on  both  the  alternating  and  direct  current 
sides  of  the  machine.  The  fact  that  it  is  a  simple  test  which 
is  easily  made,  and  that  it  gives  direct  readings  without  any 
calculation,  has  led  engineers  to  often  overlook  the  fact  that  its 
accuracy  under  the  usual  conditions  of  test  is  very  doubtful. 

Measuring  instruments,  even  when  carefully  calibrated, 
cannot  be  relied  upon  to  give  readings  to  a  greater  accuracy  than 
one  per  cent  under  practical  operating  conditions,  and  the  error 
is  often  nearer  two  or  three  per  cent.  The  reason  for  this  is  that 
the  presence  of  large  masses  of  iron  or  stray  magnetic  fields  affects 
their  accuracy  very  considerably,  and  these  local  conditions  are 

119 


120  INPUT-OUTPUT  EFFICIENCY  TESTS 

continually  changing  and  rarely  the  same  for  any  two  tests.  In 
addition  to  the  inaccuracies  of  the  instruments,  errors  in  obser- 
vation also  affect  the  results,  and  it  requires  extremely  careful 
work  on  the  part  of  accurate  and  experienced  observers  to  repeat 
readings  consistently  to  one  per  cent.  As  a  result  of  this,  it  is 
very  doubtful  whether  the  readings  taken  under  practical  operat- 
ing conditions,  in  the  factory  or  in  the  sub-station,  can  be  ob- 
tained with  a  greater  average  accuracy  than  two  per  cent.  And, 
when  it  is  considered  that  this  inaccuracy  of  two  per  cent  may 
occur  in  both  the  input  and  output  readings,  resulting  in  a 
possible  error  of  four  per  cent  in  the  combined  efficiency,  we  can 
see  that  such  a  test  is  valueless  to  check  guarantees. 

An  additional  source  of  inaccuracy  in  a  commercial  input- 
output  sub-station  efficiency  test  is  that  due  to  the  power-factor 
of  any  synchronous  alternating  current  machine  tested,  being 
other  than  100  per  cent,  and  that  due  to  the  increased  loss  caused 
by  inaccurate  setting  and  poor  condition  of  the  brushes  in  a 
direct  current  machine.  There  are  a  number  of  such  conditions 
which  affect  the  efficiency  as  tested  by  this  method,  which  are 
apt  to  be  overlooked,  or  which  are  controlled  with  difficulty 
under  practical  conditions. 

When  we  contrast  the  above  objections  with  those  which 
can  be  raised  against  the  separate  loss  method,  we  realize  that 
the  extra  complication  of  this  method  is  justified  by  the  more 
accurate  results  obtained.  In  the  separate  loss  method  each 
individual  loss  is  measured  separately,  and  a  possible  error  of  two 
or  three  per  cent  in  the  measurement  of  the  losses,  will  usually 
not  make  an  error  of  more  than  0.25  per  cent  in  the  combined  effi- 
ciency. Each  loss  can  be  measured  under  favorable  conditions, 
or  under  the  particular  conditions  which  are  being  considered, 
and  when  the  test  is  completed  there  can  be  no  suspicion  that 
the  accuracy  of  the  results  has  been  influenced  by  unknown 
factors.  Separate  measurement  of  the  individual  losses  also 
shows  at  once  how  the  losses  are  distributed,  and  allows  the 
operating  characteristics  of  the  machine  under  various  con- 
ditions of  load  to  be  defined  more  accurately. 


INPUT-OUTPUT  EFFICIENCY  TESTS  121 

The  input-output  method  of  testing  rotary  converters  and 
motor  generators  has  long  been  considered  convenient  and 
sufficient  by  operating  engineers,  on  account  of  the  ease  with 
which  readings  can  be  obtained.  But,  from  what  has  been  said 
above,  it  will  be  realized  that  the  additional  time  and  expense 
of  measuring  the  efficiency  by  the  separate  loss  method  is  well 
justified  by  the  more  accurate  results,  and  by  the  additional 
information  obtained  in  regard  to  the  characteristics  and  opera- 
tion of  the  machine.  The  input-output  method  can  be  used  as 
a  rough  check  in  cases  where  accuracy  is  not  of  importance,  but 
in  all  cases  where  an  accurate  efficiency  test  is  required  there 
seems  to  be  little  doubt  that  the  separate  loss  test  is  the  only  one 
which  can  be  relied  upon. 


[Presented  before  the  National  Electric  Light 
Association,  May  26,  1910.] 


DIRECT  CURRENT  TURBO  GENERATORS 


Direct  Current  Supply. — Large  lighting  and  railway  central 
stations  supplying  power  to  direct  current  systems  are  gradually 
abandoning  the  use  of  direct  current  generating  apparatus,  and 
are  instead  generating  alternating  current,  which  is  transformed 
to  direct  current  after  distribution  to  sub-stations.  But  the 
number  of  small  power  stations  and  isolated  plants  generating 
direct  current  is  increasing  yearly,  while  the  use  of  direct  current 
for  excitation  of  alternators,  or  for  train  lighting,  also  creates  a 
large  demand  for  small  and  medium  size  direct  current  gener- 
ators. It  has  long  been  recognized  that  this  demand  is  most 
satisfactorily  met  by  some  form  of  direct  current  steam  turbine 
driven  set.  The  difficulty  in  building  such  a  unit  is  to  design  a 
direct  current  generator  which  will  operate  satisfactorily  at  the 
high  speed  required  for  the  economical  operation  of  the  steam 
turbine. 

The  advantages  of  the  steam  turbine  set  are  smaller  floor 
space  and  lower  maintenance,  due  to  absence  of  reciprocating 
parts.  And  as  these  advantages  are  often  relatively  important, 
several  methods  have  been  employed  for  obtaining  direct  current 
from  a  steam  turbine  driven  generator.  The  most  important 
of  these  are: 

1.  A  direct  current  generator  direct  connected  to  the  steam 
turbine,  and  designed  to  operate  satisfactorily  at  the  high  speed 
required. 

2.  A  unipolar  generator. 

123 


124  DIRECT   CURRENT  TURBO   GENERATORS 

3.  An  alternating  current  synchronous  or  induction  generator 
direct  connected  to  the  steam  turbine,  and  supplying  current  to 
a  rotary  converter  which  transforms  the  alternating  to  direct 
current. 

The  rotary  converter  method  has,  up  to  the  present  time, 
proved  to  be  the  most  conservative  arrangement,  and  prob- 
ably will  continue  so  for  large  units.  The  unipolar  generator  has 
been  used  in  a  few  special  cases,  but  both  it  and  the  combined 
rotary  and  alternating  current  generator,  are  at  the  present  time 
being  gradually  displaced  by  the  direct-driven  direct  current 
turbo  generator  for  all  small  capacity  units. 

Historical. — Direct  current  turbo  generators  have  been  built 
in  Europe  for  the  past  fifteen  years,  but  then-  design  and  opera- 
tion has  not  until  recently  been  sufficiently  satisfactory  for  them 
to  be  considered  under  American  conditions.  The  early  direct 
current  turbo  generators  were  built  with  smooth  core  armature 
and  copper  brushes,  so  that  the  operation  was  poor.  Recently, 
however,  several  European  manufacturers  have  built  direct 
current  turbo  generators  in  sizes  up  to  1,250  K.W.  and  4,000 
amperes  which  have  proved  much  more  satisfactory.  All  the 
machines  above  referred  to  operated  with  metallic  brushes,  and 
naturally  suffered  from  the  handicap  of  excessive  maintenance 
cost.  But  about  two  years  ago,  when  the  direct  current  turbo 
generator  was  extensively  adopted,  the  question  of  mainten- 
ance became  so  serious,  that  operating  engineers  took  matters 
into  their  own  hands  and  insisted  on  replacing  the  metallic 
brushes  by  high-grade  carbon  or  graphite  brushes.  In  a 
number  of  instances  these  European  direct  current  turbo 
generators,  which  were  originally  built  and  shipped  from  the 
factory  to  operate  with  metallic  brushes,  have,  because  of  the 
difficulty  of  keeping  this  brush  gear  in  running  condition,  been 
modified  after  installation,  so  as  to  operate  with  carbon  brushes. 
The  result  of  this  has  been  that  European  manufacturers  are  now 
adapting  their  machines  where  possible  to  operate  with  carbon 
brushes.  American  manufacturers  realized  early  that  the  direct 
current  turbo  generator  would  never  be  a  satisfactory  commercial 


DIRECT  CURRENT  TURBO   GENERATORS  125 

machine  until  it  could  be  built  with  carbon  brushes.  But  it  is 
only  within  the  last  five  years,  that  the  skill  of  designers  and 
manufacturers  has  been  equal  to  constructing  direct  current 
generators  to  operate  satisfactorily  with  carbon  brushes  at 
speeds  materially  higher  than  those  of  the  standard  belt-driven 
generator.  By  careful  design  with  auxiliary  commutating  poles 
and  by  accurate  shop-work  it  has  been  possible  to  build  motor- 
generators  for  doub  e  the  speed  which  was  formerly  considered 


FIG.  28.— 50  K.W.,  125  Volt,  3,000  R.P.M.,  D.C.  Turbo  Generator. 

possible,  while  the  development  of  direct  current  generators  for 
coupling  to  steam  turbines  has  proceeded  under  the  same  con- 
ditions. It  can  now  be  considered  that  the  direct  current 
turbo  generator  has  been  developed,  suitable  for  satisfactory 
operation  with  carbon  brushes  under  American  conditions.  They 
are  being  built  in  sizes  from  10  to  300  kilowatts  at  125  volts, 
and  from  50  to  500  kilowatts  at  250  volts,  while  designers  are 
working  on  still  larger  units.  Although  600  volt  generators  do 
not  seem  to  be  in  a  great  demand  for  this  type  of  unit,  a  number 


126  DIRECT  CURRENT  TURBO   GENERATORS 

have  been  in  satisfactory  operation  for  some  time,  notably  a 
1,000  K.W.  unit  (consisting  of  two  500  K.W.  generators  coupled 
to  one  steam  turbine),  operating  at  1,500  R.P.M.,  which  was  in- 
stalled by  the  North  Shore  Railway  Company,  California,  in  1907. 
Designs. — The  following  is  a  list  of  approximately  standard 
speeds  which  have  been  found  most  suitable  for  these  generators: 

K.W.  R.P.M.  Volts. 

10  6000  125 

25  4500  125 

50  3000  125  and  250 

75  2800 

100  2400 

150  2200 

200  2000 

300  1800 

500  1500  250 

All  of  which  machines  can  be  satisfactorily  built  to  operate  with 
commutating  poles  when  carefully  designed.  At  one  time  en- 
gineers considered  that  a  commutating  pole  of  almost  any  design 
was  a  universal  remedy  for  all  commutating  troubles,  but  ex- 
perience with  direct  current  turbo  generators  and  other  high 
speed  machines,  has  shown  that  this  is  very  far  from  being  true. 
The  commutating  pole  must  be  proportioned  as  carefully  as  the 
other  parts  of  a  machine;  and  it  was  the  neglect  of  this  fact 
which  caused  the  failure  and  abandonment  of  this  device  when 
first  used  many  years  ago. 

The  two  factors  which  limit  the  design  of  direct  current  turbo 
generators  are  the  commutation,  and  the  collection  of  large 
currents  at  high  speeds.  The  commutating  difficulties  can  be 
satisfactorily  overcome  in  the  generators  given  in  the  above  list, 
if  a  properly  designed  interpole  construction  is  used,  though  for 
generators  of  more  special  or  more  extreme  ratings  a  complete 
system  of  distributed  compensating  winding  in  the  pole-faces  is 
usually  necessary.  The  limiting  speed  for  which  it  is  possible  to 
build  large  direct  current  turbo  generators  is  decided  by  the 
maximum  commutator  peripheral  speed,  which  can  be  conserva- 


DIRECT   CURRENT  TURBO   GENERATORS  127 

tively  operated  with  the  particular  grade  of  brushes  adopted. 
The  following  equation  limits  the  design  of  the  commutator: 

Commutator     peripheral      speed  =  Circumferential      distance      between 
brushes  on  the  commutator  X  the  number  of  poles  X  R.P.M. 

The  minimum  distance  between  brush-arms  on  the  commutator, 
which  can  be  conservatively  allowed  for  any  given  voltage,  is 
definitely  fixed  by  the  mechanical  clearance  necessary  for  accessi- 
bility and  to  prevent  flashing,  and  by  the  space  required  for  the 
necessary  number  of  commutator  segments  per  pole.  The  niun- 


FIG.  29.— 100  K.W.,  12.5  Volt,  2,400  R.P.M. ,  B.C.  Turbo  ( iem-rator. 

ber  of  poles  is  decided  by  the  current  the  machine  is  to  coin- 
mutate.  The  maximum  commutator  peripheral  speed  is  de- 
cided by  the  standard  of  workmanship  and  by  the  type  or  quality 
of  carbon  brushes  adopted;  while  the  revolutions  per  minute 
should  be  fixed  by  the  question  of  maximum  economy  for  the 
steam  turbine.  Thus  we  have  the  above  equation  stating  a 
relation  between  a  number  of  factors,  each  one  of  which  is  subject 
to  restriction. 

As  an  example  of  this  we  can  consider  a  500  K.W.,  250  volt 
direct  current  turbo  generator,  to  operate  with  a  commutator 


128  DIRECT   CURRENT  TURBO    GENERATORS 

peripheral  speed  of  5,500  feet  per  minute.  We  have  2,000 
amperes  to  commutate,  which  will  require  at  least  four,  and 
preferably  six  poles.  The  minimum  allowable  distance  between 
brushes  on  the  commutator  for  a  machine  of  this  size  and  type 
is  about  7  or  8  inches,  while  preferably  it  should  be  10  or  12 
inches.  The  more  conservative  figure  would  result  in  a  14-inch 
diameter  commutator  at  1,500  R.P.M.  for  a  four-pole  machine, 
or  a  21-inch  commutator  at  1,000  R.P.M.  for  a  six-pole  machine, 
though  adopting  7J  inches  distance  between  brushes  we  could 
operate  the  six-pole  machine  at  1,500  R.P.M. ,  using  a  14-inch 
diameter  commutator.  But  adopting  the  more  conservative 
figure,  and  a  four-pole  machine  at  1,500  R.P.M.,  we  must  com- 
mutate 1,000  amperes  per  pole,  which  will  require  approximately 
twenty-six  1 J"  x  I"  brushes.  Thus  we  will  require  a  commutator 
14  inches  in  diameter  and  approximately  56  inches  long,  or  two 
commutators  each  14  inches  in  diameter  and  28  inches  long. 
This  example  shows  the  difficulties  in  operating  large  direct 
current  turbo  generators  at  high  speeds  when  a  conservative 
design  is  followed ;  and  explains  also  why  generators  of  extreme 
rating,  in  regard  to  voltage  or  current,  become  so  difficult  to 
build. 

Commutator  and  Brush-Gear. — On  account  of  the  careful  de- 
sign and  accurate  shop  work  required,  the  question  of  collecting 
large  currents  at  high  speed  with  carbon  brushes  is  the  most 
difficult  problem  in  connection  with  the  design  and  manufacture 
of  direct  current  turbo  generators.  Flexible  metallic  brushes 
will  operate  whether  the  commutator  runs  true  or  not;  while 
having  low  contact  resistance,  they  are  suitable  for  collection  of 
large  currents;  and  this  explains  why  they  were  adopted  univer- 
sally on  the  early  European  machines.  The  difficulty  in  operat- 
ing with  this  type  of  brush  is  due  to  the  fact  that  it  is  almost 
impossible  to  entirely  eliminate  sparking,  unless  carbon  trailing 
tips  are  used ;  while  it  is  necessary  to  keep  the  brushes  carefully 
trimmed  if  the  operation  is  to  be  at  all  reliable.  If  the  brushes 
are  not  trimmed  frequently,  the  trailing  edge  of  a  copper  gauze 


DIRECT   CURRENT  TURBO   GENERATORS  Hi) 

or  wire  brush  becomes  ragged,  and  when  the  brush  is  in  such 
condition  a  short-circuit  or  sudden  violent  change  in  load  is 
liable  to  make  the  machine  flash  over.  A  new  typo  of  copper- 
leaf-graphite  brush  has  been  used  recently  in  Kurope  with  better 
results,  but  the  operation  cannot  be  considered  satisfactory,  and 
the  cost  is  high.  Carbon  trailing-tips  have  been  used  with 
metallic  brushes,  but  this  results  in  a  complicated  and  sensitive 
brush-gear,  which  is  almost  as  difficult  to  manufacture  and  keep 
in  operative  condition  as  a  brush-gear  using  entirely  carbon 
brushes.  The  only  reason  for  the  adoption  of  a  carbon  trailing- 
tip  and  copper  brush  combination  is  that  it  makes  possible  the 
use  of  a  smaller  commutator  than  would  be  necessary  with  all 
carbon  brush-gear.  In  addition  to  the  excessive  attention  re- 


FIG.  30.— Armature  for  200   K.W.,   ll>r>  Volt,   1,800  R.P.M.,   D.C. 
Turbo  (Joiu'rator. 

quired,  the  life  of  metallic  brush-gear  of  all  types  is  so  very  short 
that  the  cost  of  maintenance  usually  becomes  prohibitive;  and 
this  is  the  main  reason  why  engineers  consider  that  the  only 
satisfactory  solution  of  the  commutator  problem  on  a  direct 
current  turbo  generator  is  the  use  of  carbon  or  graphite  brushes. 
The  better  the  quality  of  these  brushes,  the  more  satisfactory 
the  operation,  but  the  greater  the  difficulty  of  obtaining  span- 
brushes  for  renewals.  It  is  an  open  question  whether  it  is  better 
commercially  to  design  these  machines  to  operate  with  ordinary 
good  quality  graphitic  carbon  brushes,  or  with  some  special  high 
grade  imported  brush.  There  is  no  question,  however,  but  that 
the  direct  current  turbo  generator  should  have  only  carbon  or 
graphite  brushes  if  it  is  to  give  satisfactory  commercial  service 
under  American  conditions. 
9 


130  DIRECT   CURRENT   TURBO   GENERATORS 

The  question  of  good  operation  with  carbon  brushes  is  a 
mechanical  one,  and  requires  a  commutator  which  runs  abso- 
lutely true  under  all  conditions  and  at  all  times.  Commutators 
to  carry  a  large  current  at  high  speed  are  always  relatively  small 
in  diameter  and  long.  The  diameter  is  fixed  by  the  revolutions 
per  minute,  and  the  maximum  peripheral  speed  which  can  be 
satisfactorily  operated  with  the  particular  grade  of  carbon  used, 
and  with  the  degree  of  accuracy  obtainable  in  the  commutator 
manufacture.  The  peripheral  speed  usually  adopted  at  the 
present  time  in  America  is  from  4,500  to  6,000  feet  per  minute, 
although  peripheral  speeds  40  per  cent  higher  than  this  have 
been  used  by  European  manufacturers  with  a  special  grade  of 
brush.  With  the  diameter  fixed,  the  length  of  the  commutator 
is  decided  by  the  questions  of  temperature  rise  and  correct  spac- 
ing of  the  requisite  number  of  brushes.  When  the  length  of  the 
commutator  is  greater  than  about  30  inches  it  becomes  usually 
advisable  to  build  two  commutators  of  half  the  length  instead 
of  one  of  full  length.  These  two  commutators  can  be  arranged 
either  one  at  each  end  of  the  armature,  or  both  in  tandem  at  one 
end  of  the  armature;  in  the  latter  the  bars  of  the  two  commuta- 
tors being  connected  by  suitable  lugs.  A  difficulty  wilich  is  ex- 
perienced with  any  long  commutator,  or  with  two  commutators 
in  tandem,  is  the  lack  of  uniformity  in  the  distribution  of  current 
between  the  different  brushes  on  each  brush-arm.  With  two 
commutators,  one  at  either  end  of  the  armature,  we  have  diffi- 
culty in  distributing  the  current  equally  between  the  two  com- 
mutators; this  difficulty  being  especially  marked  if  a  single 
armature  winding  instead  of  two  independent  windings  is  used. 
The  difficulty  in  obtaining  uniform  distribution  of  current  is 
about  the  same  with  each  of  these  three  types  of  commutators, 
and  it  can  be  avoided  only  by  selection  of  a  suitable  type  of 
brush-holder,  with  good  quality  brushes  of  uniform  quality, 
and  a  suitable  arrangement  of  generator  leads. 

The  standard  construction  adopted  for  direct  current  turbo 
generators  is  a  cylindrical  commutator  of  the  shrink-ring  type. 
Radial  commutators  have  been  used  to  a  certain  extent  in 


DIRECT    CURRENT  TURBO   GENERATORS  131 

Europe  with  good  results,  but  the  inaccessibility  of  the  brushes 
has  prevented  their  extensive  adoption.  The  only  advantage 
claimed  for  them  is  a  reduction  in  over-all  length,  and  less  trouble 
due  to  vibration  of  the  armature  caused  by  lack  of  balance. 
This  latter  advantage  is  due  to  the  fact  that  the  operating  surface 
of  the  commutator  is  at  right  angles  to  the  shaft,  and  consequent- 
ly in  the  same  plane  as  any  vibration,  instead  of  being  at  right 
angles  to  such  a  plane,  as  is  the  case  with  a  cylindrical  commu- 
tator. The  standard  cylindrical  shrink-ring  type  of  commutator 
is  in  small  sizes  built  directly  on  the  shaft,  while  in  large  sizes  it 
is  built  on  a  bushing.  The  success  or  failure  of  a  commutator 
depends  upon  extremely  accurate  shop  work,  and  on  the  adop- 
tion of  a  design  such  that  the  deflections  and  stresses  due  to 
centrifugal  action  and  temperature  variation  are  moderate. 
Accurate  and  experienced  shop  work  is  the  foundation  of  all 
good  operation  in  direct  current  turbo  generators,  and  it  is  this 
education  and  development  of  the  shop  as  much  as  anything 
else  which  has  rendered  this  type  of  generator  possible. 

The  manufacture  of  high-speed  commutators  differs  from  that 
of  the  corresponding  low  speed  in  that  much  greater  accuracy 
is  required.  The  micanite  or  mica  used  in  the  construction,  in- 
stead of  being  a  heterogeneous  combination  of  mica  and  shellac, 
must  be  built  up  of  carefully  gauged  and  selected  mica  segments 
of  uniform  thickness  regularly  arranged  and  cemented  together 
with  the  minimum  amount  of  shellac.  This  micanite  has  to  be 
suitably  treated  so  that  it  takes  its  final  dimensions  before  being 
placed  in  the  commutator.  Every  element  in  the  commutator, 
that  is,  the  copper,  micanite,  bushing  and  shrink-rings,  must  be 
accurately  gauged,  and  after  the  commutator  is  assembled  it 
must  also  be  carefully  seasoned,  so  that  there  remains  no  possibil- 
ity of  distortion  or  of  change  in  the  relative  position  of  segments 
after  the  machine  is  placed  in  operation.  Variation  in  tempera- 
ture and  mechanical  stresses  are  the  primary  causes  of  commu- 
tator mechanical  trouble,  and  the  more  perfect  the  commutator 
the  better  will  it  stand  these.  The  Y-ring  type  of  commutator 
is  unsuitable  for  long  high-speed  commutators  of  small  diameter, 


132  DIRECT   CURRENT  TURBO   GENERATORS 

as  with  this  construction  it  is  difficult  to  keep  the  mechanical 
stresses  within  reasonable  limits,  and  the  advantage  of  the 
shrink-ring  construction  is  that  the  stresses  can  be  directly 
calculated  and  arrangements  made  to  take  care  of  them.  The 
shrink-rings  should  be  of  high  grade  steel  of  sufficiently  heavy 
section,  so  that  the  stresses  due  to  centrifugal  action  become 
moderate.  They  must  also  be  stiff  enough  to  retain  their 
circular  form  and  to  prevent  any  local  distortion  of  the 
commutator. 

Practice  varies  in  regard  to  undercutting  the  mica  segments. 
When  soft  graphite  brushes  are  used,  undercutting  the  mica  seg- 
ments is  essential  for  good  running,  but  with  hard  brushes  it  is 
not.  Probably  the  best  results  are  obtained  on  these  high-speed 
commutators  when  graphite  brushes  and  undercut  mica  are  used. 
The  undercut  grooves  should,  however,  be  cleaned  out  occasion- 
ally to  prevent  the  accumulation  of  carbon  dust  and  dirt. 

Mechanical  Construction. — The  mechanical  construction  of 
the  armature  is  of  great  importance,  since  it  is  essential  that  the 
balance  of  the  armature  should  not  change  after  the  machine  is 
put  in  operation.  This  necessitates  that  the  punchings  do  not 
become  loose  nor  move  on  the  shaft,  and  that  the  armature  wind- 
ing does  not  move  under  the  action  of  centrifugal  forces.  The 
punchings  are  usually  either  pressed  on  the  shaft  one  at  a  time, 
or  built  up  on  a  mandril  bored  out,  and  shrunk  on  the  shaft,  no 
intermediate  spider  being  used  on  account  of  the  small  diameter. 
Opinion  varies  as  to  whether  the  armature  coils  are  better  held 
in  position  by  wedges  or  by  wire  bands,  but  the  most  satisfactory 
arrangement  seems  to  be  the  use  of  bands  on  the  small,  and 
wedges  on  large  armatures.  The  end  connections  on  the  arma- 
ture are  probably  better  held  in  position  by  steel  wire  bands. 
Bronze  rings  have  been  used  for  this  purpose,  but  there  is  danger 
that  they  may  become  loose  and  change  the  balance  of  the 
armature,  as  it  is  very  difficult  to  fix  them  securely.  The  ques- 
tion of  insulation  of  the  armature  winding  is  extremely  impor- 
tant, as  the  armature  winding  is  exposed  to  carbon  and  copper 
dust  from  the  commutator,  and  the  collection  of  dirt  on  such  a 


DIRECT  CURRENT  TURBO   GENERATORS  133 

high  speed  armature  is  very  much  greater  than  on  the  corre- 
sponding low  speed.  On  account  of  this  it  is  necessary  to  be 
extremely  careful  in  insulating  all  bare  metal  on  the  armature, 
so  that  there  will  be  no  danger  of  flashing  over  dirty  surfaces  to 
ground ;  while  the  insulation  on  the  armature  coils  must  be  care- 
fully baked  and  pressed,  so  that  there  will  be  no  shrinkage  and 
consequent  movement  of  the  coils.  The  whole  question  of  satis- 
factory armature  and  commutator  construction  lies  in  working 
out  the  numerous  details  in  design  and  manufacture,  so  as  to  ob- 
tain an  armature  and  commutator,  satisfactory  both  mechani- 
cally and  electrically  at  the  time  it  is  built,  and  so  thoroughly 
seasoned  before  put  in  operation  that  it  will  not  change  appre- 
ciably with  time. 

The  question  of  vibration  is  one  of  the  most  serious  difficulties 
to  be  considered  in  these  machines.  It  is  difficult  to  predeter- 
mine the  critical  speed  of  a  direct  current  turbo  generator 
armature;  but  it  is  very  important  that  this  critical  speed  of  the 
generator,  when  coupled  to  the  steam  turbine,  shall  not  be  close 
to  the  normal  running  speed.  This  usually  requires  that  the 
armature  must  be  designed  with  the  maximum  possible  diameter 
of  shaft,  and  it  is  generally  necessary  to  sacrifice  the  advantage 
of  low  commutator  peripheral  speed  to  enable  a  sufficiently  stiff 
shaft  to  be  used.  The  question  of  permanency  of  balance  is 
equally  important,  and  this  requires  that  there  be  no  relative 
movement  of  the  component  parts  of  the  armature  with  time, 
and  also  that  the  shaft  neither  spring  nor  deflect  under  the  in- 
fluence of  the  temperature  variations  obtained.  Direct  current 
turbo  generators  as  built  a  few  years  ago  would  operate  perfectly 
on  test,  when  first  built,  but  after  running  six  months  mechanical 
vibration  and  deterioration  of  commutator  were  frequently  so 
great  that  they  could  no  longer  be  considered  commercial. 

One  of  the  most  satisfactory  constructions  for  small  units  is 
a  two  bearing  set,  the  turbine  wheel  being  overhung  and  the  two 
bearings  self-aligning;  as  this  construction  obviates  any  trouble 
due  to  lack  of  alignment.  With  larger  units,  however,  it  is  no 
longer  suitable  on  account  of  the  axial  space  required  by  the  tur- 


134  DIRECT  CURRENT  TURBO   GENERATORS 

bine,  and  a  three  or  four  bearing  set  with  a  coupling,  preferably 
a  rigid  one,  beomes  necessary.  Such  sets  again  require  careful 
alignment  and  careful  fitting  of  the  coupling  and  bearings ;  other- 
wise there  will  be  trouble  with  vibration.  Oil-ring  lubrication  is 
effective  in  the  smaller  sizes,  but  forced  flow  lubrication  is  usually 
required  in  capacities  above  50  K.W.,  if  the  temperature  of  the 
bearing  is  to  be  kept  within  reasonable  limits  and  operation  to 
be  reliable. 

Foreign  practice  is  usually  to  completely  enclose  the  arma- 
ture, except  the  commutator,  and  to  supply  cooling  air  from  a 
special  duct.  This  is  hardly  considered  good  American  practice 
on  account  of  the  difficulty  of  access,  and  usually  on  small 
machines  a  semi-enclosed  construction  with  natural  cooling  is 
adopted.  On  large  machines,  however,  as  the  noise  is  appre- 
ciably more  than  in  corresponding  low-speed  units,  it  may 
ultimately  be  found  advisable  to  adopt  a  more  enclosed 
construction. 

Present  Situation. — At  the  present  time  the  direct  current 
turbo  generator  can  hardly  be  considered  as  commercially  suit- 
able for  the  American  market  above  500  K. W.  at  250  volt ;  and 
the  probability  is  that  in  larger  sizes  it  will  be  necessory,  for  the 
present,  to  use  an  alternating  current  turbo  generator  and  rotary 
converter  as  a  substitute,  though  this  substitute  may  be  only 
temporary  in  the  750  K.W.  and  possibly  the  1,000  K.W.  sizes. 
Considerably  larger  sizes  are  at  present  in  use  in  Europe,  but  it 
should  be  remembered  that  operating  conditions  there,  are  by 
no  means  as  severe  as  they  are  here.  A  typical  example  of 
European  direct  current  turbo  generator  installation  which  was 
recently  inspected  by  the  writer  on  a  large  steamship,  exemplifies 
this  latter  point.  It  consisted  of  four  units  operating  with  me- 
tallic brushes;  the  normal  load  being  sufficient  only  to  fully  load 
two  machines,  and  the  load  being  changed  around  from  one  unit 
to  another.  The  generators  after  operating  six  days  in  this  way 
were  subject  to  three  or  four  days'  overhauling  while  the  boat 
was  in  port,  which  overhauling  consisted  in  replacing  the  brushes 
with  a  newly  trimmed  set,  and  in  carefully  sand-papering  the 


DIRECT  CURRENT  TURBO   GENERATORS  135 

commutator  and  adjusting  the  brushes.  With  this  attention  the 
units  gave  very  good  satisfaction,  but  it  is  obvious  that  such 
results,  and  they  are  to  be  expected  from  the  use  of  metallic 
brushes,  make  these  machines  unsuitable  for  the  American 
market.  It  is  the  necessity  of  developing  direct  current  turbo 
generators  capable  of  operating  with  carbon  brushes  and  a 
minimum  amount  of  attention,  that  has  caused  American  manu- 
facturers to  delay  in  placing  this  type  of  machine  on  the  market. 
At  the  present  time  such  units  can  be  considered  commercial  in 
the  smaller  sizes,  while  there  is  the  possibility  of  larger  units  being 
developed  in  the  future. 


V- 


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